This invention provides an improved process for the conversion of reactant into a reaction product, in the presence of a solid acid catalyst comprising sulfonic acid groups covalently bonded to a polymeric chain, wherein the improvement comprises increasing the rate of conversion, on an equivalent sulfonic acid basis, by providing, as said polymeric chain a compound represented by the general formula:M(O.sub.3 ZO.sub.x R).sub.nwherein M is a tetravalent metal ion; Z is a pentavalent atom, selected from the group consisting of elements of Group V of the Periodic Table of the Elements having an atomic weight greater than 30; x varies from 0 to 1; R is selected from the group consisting of organo radicals and mixtures of hydrogen radicals and organo radicals; and n varies from 1 to 2; provided that n is 1 when R is terminated with a tri-or tetraoxy pentavalent atom.