The acid strength of solid acid materials can be assessed using the linear free energy relationship, applying the Hammett-Brown equation (logk(X)/k(H) = rho sigma(+)) for the H/D exchange with substituted benzenes. The magnitude of the sigma varies for different solid acid materials (Amberlyst-15, zeolite HUSY, K-10 montmorillonite, and niobic acid) and is associated with the degree of proton transfer in the transition state. The results were compared with the rho values obtained for H/D exchange with sulfuric acid solutions, indicating that neither solid acid is stronger than 98% sulfuric acid solution. Calculations also revealed that H/D exchange is mostly concerted, with little degree of charge development in the aromatic ring in the transition state. (c) 2007 Elsevier Inc. All rights reserved.