Several conditions for the sol-gel polymerisation of hybrid silica-zirconia and silica-titania have been studied using specially tailored electrochemical probes. One of the probes has been designed to bind to the silica backbone, while the other has been designed to bind to the transition metal species. The results show that the silicon and the transition metal species play a quite different role in the polymerization course, and especially large differences between the zirconium and the titanium behaviour have been observed. It is shown unambiguously that while the zirconia species bind irreversibly to the silica gel, the titania, ones remain very mobile whatever the advancement of the reaction and probably act only as catalysts for the siloxane polycondensation. (c) 2006 Published by Elsevier B.V.