The effects of addition of alumina to the matrices of cracking catalysts containing different types of zeolite Y, on their cracking performance, were investigated using a micro activity test and two different feed oils. For the heavier feed oil, the alumina addition resulted in a higher conversion at the same catalyst to oil ratio independent of the type of zeolite. This higher conversion was accompanied by a greater selectivity for coke and a lower selectivity for gasoline. For the lighter feed oil the effect of alumina addition on the total conversion was much less pronounced while the effects on the selectivity were similar to those observed using the heavier feed. The performance of the catalysts in a commercial fluid catalytic cracking unit is discussed in view of their coke forming tendencies and the heat balance of the cracker.