The catalytic cracking of gasoil has been carried out on a MCM-22 zeolite sample. This zeolite, in agreement with the proposed structure, shows little activity for cracking the large molecules present in the vacuum gasoil feed. However, when it is used as a cracking additive in combination with a USY based cracking catalyst, and its behaviour is compared with that of a ZSM-5 zeolite additive, it is found that MCM-22 produces less total gases, as well as less dry gases, with a lower loss of gasoline than ZSM-5. On the other hand, the propylene to propane and butene to butane ratio is higher in the case of MCM-22, showing a very adequate behaviour within the frame of the production of reformulated gasolines. However, ZSM-5 is more active than MCM-22 and at low rates of addition, similar to 3 wt%, ZSM-5 shows a larger effect than MCM-22. In order to see the benefits of MCM-22 as a cracking additive, larger amounts (similar to 13 wt%) need to be introduced. ZSM-5 gives a higher ratio of protolytic to beta-scission cracking than MCM-22, and the steam stability of MCM-22 is quite good, being even higher than that of ZSM-5. Finally, we present a method for testing additives during gasoil cracking which enhances their effects and allows us to show very clearly the cracking behaviour of zeolite additives.