For applications such as integrated optical devices, it is desirable to fabricate thick optical films, e.g. by the sol-gel process. while avoiding some of the problems associated with multilayer deposits. In the present work, alkoxide-derived 5i02 films were deposited by spin-coating onto single crystal silicon wafers, while varying independently several experimental parameters, namely the water/tetraethoxysilane (TEOS) molar ratio R, the volume percentage of TEOS, the aging period of the solutions and the rotational speed of the substrate. It was concluded that the thickness of the as-deposited porous silica gel films increased with decreasing R, as well as with increasing TEOS volume percentage or aging period of the solution (up to a certain point), whereas it showed a maximum when plotted as a funtion of the substrate rotational speed. Maximum thicknesses in excess of 370 urn were obtained for porous films, for which densification did not cause cra.cking,while porous undensified films up to 1.2 pm thick were also prepared. The conditions under which maxiimiin film thickness is achieved are discussed.