The effect of mixing of an ultrastabilised acid form of Y zeolite (USHY, Si/Al=4.5) with two different HZSM-5 zeolites, synthesised by different methods, with template (CVN, Si/Al=45) and without template (CVM, Si/Al=26), and presenting quite different acidic properties, is studied using as model reaction the transformation of n-heptane at 350 degrees C. The results obtained point to the existence of some interactions between the processes occurring in the two zeolites, and are helpful in the understanding of the known ZSM-5 addition effect in the cracking catalysts. There is a clear enhancement in the formation of ramified C-4 products for the USHY:ZSM-5 mixtures (75.25, wt%) and (50:50), relative to the additive linear predictions, this effect being significantly more extensive for the mixtures with the ZSM-5 sample presenting the highest acid strength. In the mixtures (90:10) there is also an increment in the ramified/linear products ratio. The initial olefin/paraffin products ratio registered a slight increase, relative to the expected values, in all the mixtures with CVN. In the case of CVM there was also an increase of this parameter, but only for the mixture with 10% of ZSM-5, a significant reduction being observed in all the other cases.