In this paper the relation between activity and acidity in a variety of ZSM-5 zeolite catalysts, with different Si/Al ratios and different protonic content, is analysed and a quantitative correlation is obtained. The acid site strength distribution was estimated using temperature-programmed desorption (TPD) of ammonia by applying a digital deconvolution method to the curves. These data were then correlated with experimental catalytic activity data fbr the same catalysts towards n-heptane cracking reaction, by means of a Bronsted-type equation similar to the ones used for homogeneous acid catalysis and already used for other zeolites. It can be noticed that the same types of equation that are used for homogeneous acid catalysis also hold for heterogeneous acid catalysis and that the activation energy for ammonia desorption can be used as acid-strength scale for the purpose of correlation with catalytic activity.