The acid strength of Pt-HBeta and Pt-HMordenite zeolite catalysts were estimated by H/D exchange of substituted benzenes with the use of linear free-energy relationship, through the use of the Hammet-Brown equation. The results indicated that compared with the parent acidic zeolite, the incorporation of Pt metal leads to a significant decrease in the acid strength, estimated by the slope (rho) of the logarithm of the relative rate (k (X)/k (H)) with the sigma(+) constant of the substituent, indicating a lower degree of proton transfer to the ring in the transition state. The results support the model of metal-support interaction in Pt-zeolite catalyst, with formation of a metal-proton adduct.