Abstract A co-entrapped mixture of [Rh (cod) Cl] 2 and Na [Ph 2 P-3-(C 6 H 4 SO 3)] within a silica sol-gel matrix modified with ca. 5% of 1-butyl-3-[3-(trimethoxysilyl) propyl] imidazolium chloride catalyzes, in n-heptane, the hydroformylation of a variety of vinylarenes. At 50 C and under 6.9 bar each of H 2 and CO the reaction is high-yielding and highly selective. Non- hindered substrates give> 95% of branched aldehydes and only< 5% of the linear isomers ...