Catalysts prepared by impregnation of the molecular species H(CO)Rh[P(C6H5)(3)](3) or ClRh[P(C6H5)(3)](3) with the aluminophosphate micropore VPI-5 have been found to catalyse the conversion of sodium 4-styrenesulfonate in aqueous solution in the presence of gaseous H-2/CO mixture into the expected aldehyde hydroformylation product together with a substantial quantity of olefin polymer. Reaction in the presence of H-2 in the gas phase gave the expected olefin hydrogenation product but no polymer, while in the presence of CO or N-2 in the gas phase no reaction product of any sort could be found. The addition of a small quantity of hydroquinone as a radical scavenger to the reaction in the presence of gaseous H-2/CO had no effect, indicating that the polymerization process did not occur by a free radical mechanism. It is suggested that polymerization occurs via a coordinated ligand insertion mechanism, and evidence suggests that the catalytic sites are those rhodium complex molecules which are externally accessible either at VPI-5 pore mouths or otherwise are located at the VPI-5 external surface, and which have been modified, probably by a ligand exchange process involving VPI-5 framework atoms.