The presence of H3PO3 in the synthesis of zirconium diphosphonates in dimethylsulfoxide, other than the expected dilution of the pillars, which in turn gives rise to a certain degree of microporosity, may also induce the formation of a high percentag. of mesopores. E.g., compounds with high surface area(300–500 m2 /g) of which 60–70% is due to mesopores with diameters 20–40 Å and the remaining is due to micropores, were obtained. It was furthermore found that the distribution of the mesopores was fairly narrow. Compounds with micropores <3 Å and mesopores of 20–30 Å behave in practice as pure and narrow mesoporous solids;they are therefore of great interest as new molecular inorgano-organic sieves.