Disclosed is a solid catalyst functionalized with sulfonic groups, containing from 20 to 99% by weight of silica, with the residual percentage being definable by means of the formula:(ZrP.sub.2 O.sub.7).sub.1-x {(Zr(HPO.sub.4).sub.2-y.[O.sub.3 P-R(SO.sub.3 H).sub.n ].sub.y }.sub.xwherein:R is an either linear or branched linear hydrocarbon radical containing from 1 to 10 carbon atoms, or an aryl radical;x is a numeral higher than 0 and of up to 1;y is a numeral higher than 0 and of up to 2;n is either 1 or 2;with said catalyst furthermore showing silicon-oxygen-phosphorus bonds on N.M.R. examination and being amorphous or substantially amorphous when examined under X rays.The catalyst is useful in the acidic catalysis, for example in reactions of hydrocarbon conversion, such as alkylation, isomerization and oligomerization. The product according to the present invention is furthermore useful as an ion exchange agent.