In immobilizing the rhodium complexes [Rh(acac)(CO)(P)] (1) and [Rh(acac)(P)(2)] (2) (P = Ph2PCH2CH2Si(OMe)(3)) onto SiO2, acetylacetone is found to be released through protonation of the acac ligand by the acidic silica-OH groups. The resulting complexes [Rh(O-{SiO2}(HO-{SiO2})(CO)(P-{SiO2})] (1a) and [Rh(O-{SiO2})(HO-{SiO2})(P-{SiO2})(2)] (2a) were successfully tested with respect to their catalytic action on 1-hexene hydroformylation as well as benzene and toluene hydrogenation. The reaction outcome, viz. the formation of aldehydes versus isomerization, depends strongly on the presence and concentration of a phosphine co-catalyst. Thus, while 1a gave only a 17% yield of aldehyde in the absence of phosphines, the yield is increased to 54% in the presence of phosphinated silica P-{SiO2} or even 94% if PPh3 is added to the solution. Without extra added phosphine, both 1a and 2a effect mainly the isomerization of 1-hexene to 2-hexene. Pre-catalyst la catalyzes also the hydrogenation of benzene at 10.5 atm H-2 and 90degreesC to give cyclohexane with a TOF of 608 h(-1). (C) 2003 Elsevier B.V. All rights reserved.