Had Snape known that Sirius was innocent, would he have tried to save him, or would he have sent his old foe to the gallows (or more precisely the dementors) even if he had been innocent? I dont think he would have helped Sirius, but I can't believe that he would participate in sending an innocent person to the gallows. I think he would just have kept mum and done nothing. What do you think?

If he had known Sirius was innocent, I think he would have caught him but taken him to Dumbledore instead. And then let Dumbledore decide what to do with him. Even if Snape hated Sirius, I don't think he would have let the Dementors give him the kiss.
But in the book, Snape thought Sirius had murdered people... so he did what he did in the book.

Snape vs Sirius

Hate each other!

Even if snape
knew that he
was inocent
he probley
would still have
called the
dementors!
Hello they hate
each other!!
Plus Lupin was
there! he would
have called them
on Lupin for being
a werewolf without
his permision!
"Laugh!"

Please dont
ask why I wrote
that why I just felt
like it!

I think he would have taken Sirius to Dumbledore instead. I mean he goes to Grimmauld Place often enough while Sirius is there but he never tries to kill him then.

Oh..a good question.
..Just like what everyone have mentioned, she might really hate Sirius, but if he knows Sirius is innocent, maybe he would have taken him to Dumbledore instead. But as in the book, it seems like he thought Sirius had murdered people, so, hating him and the thought that he wasn't innocent at all,probably,he might have sent Sirius to the Dementors and let his soul be sucked out..

Remember, Snape was following the trio to the Whomping Willow. So he most likely heard the story of how Sirius was framed, and when he surprised them with his presence he was not a bit torn about hearing the news. So I don't think Snape would have saved him at all. In fact, I think that Snape would still have, if he had had his way, marched Sirius up to the gallows himself. Snape can hold a grudge, and even if Sirius is innocent, a lot of bad blood has grown between them. Snape can't shrug that off just because Sirius is innocent about one thing. Because he's not innocent of so many other things, in Snape's mind.

I think he'd take Sirius to Dumbledore and do everything to persuade Dumbledore that Sirius is still bad.

There has always been alot of bad blood between Snape and Black. After all Black sent Snape into the passage after Lupin, knowing what was at the other end. If it hadn't been for James Snape would have been killed. Although I think that if Snape had known that Black was innocent of the crimes he would have taken him to Dumbledor instead of handing him over to the Dementors. Of coarse he didn't know this. If it hadn't been for Harry Snape would have handed him over to suffer the death eaters kiss. Probably out of pure bitterness and hatred.

If he'd have known he probably still would have caught him but he didnt know because the 3rd book says the only people that knew the truth were harry ron hermione lupin sirius and petigrew. and dumbledore

I think Snape would have handed Sirius over to the Dementors.
I also believe Snape was the one to tell Lucius that Sirius could transform into a dog.
I would love to hear the story of the Marauders at Hogwarts, because I have always felt Snape hated Sirius more than he hated James. His reactions when Sirius escapes again is completely mental. He is willing to throw away all Dumbledores protection of him and have Harry expelled.
Everytime I read the 7th book I go softly towards Snape, but then I start reading the books all over again and I see him for that evil person he is deep inside.

Well... Snape really disliked Sirius. But even so, I think he would have grudgingly taken him to Dumbledore instead of the Dementors.

I'm sad to say I believe Snape knows that Sirius is innocent, but is still willing to subject him to the Dementor's kiss, because he wants revenge for the Whomping Willow - he still can't forget that Sirius was never punished for that.

He couldn't very well have killed Sirius in Grimmauld place: DD and everybody would have known, and then Snape would have been in serious trouble. The beauty of his revenge in PoA was that no one would have blamed him - on the contrary, he was the hero of the day, Fudge even wanted to give him an Order of Merlin!

Reasons why I suspect Snape knew:

1. Snape was a Death Eeater. Sirius tells Peter how he heard other Death Eaters in Azkaban curse Peter as a traitor - because when Voldie was brought down, they thought Peter had led him to James and Lily on purpose, because he wanted Voldie dead. But if the other DEs knew it was Peter who was the traitor, then Snape must also have known this.
2. Snape heard Sirius and Lupin tell the whole story of the Secret Keeping. If you check in the text when they felt someone enter through the door (Snape under invisibility cloak) and when he takes the cloak off and reveals himself, you will see that he stood there and heard it all...

I believe Snape never said anything when Sirius was in Azkaban, and was so beside himself with anger and disappointment when Sirius escaped the kiss, because he believed Sirius deserved it all because of the Whomping Willow incident