Ex-Ayr striker Stewart Kean return to haunt his old club as Saints went marching into the quarter finals. Goals from Kean and Hugh Murray sent the Buddies into the last eight.
Kick-off was delayed by 30 minutes to allow the large travelling support into the ground, and they had plenty to celebrate. Kean struck in first half stoppage time with a left-foot shot from 16 yards.
Ayr had Darren Henderson sent off only five minutes into the second half when he was booked twice in 60 seconds.
Saints coasted it from there, and only a number of super saves from Ludovic Roy kept Ayr in the game.
He denied Murray and Allan Russell before finally being beaten again nine minutes from time.
Russell found space on the touch line and crossed for Murray to head home at the back post and finish the tie.