Silica coatings have been deposited on various alloys by MOCVD (Metal Organic Chemical Vapor Deposition) to protect them against high temperature corrosion in coal gasification environments. DiAcetoxyDitertiaryButoxySilane (DADBS) has been used as a metal organic precursor at deposition temperatures between 773 - 873 °K and amorphous layers were produced with a growth rate of about 1 μm. h-1. These coatings have been tested at 823°K in a sulphiclizing atmosphere with a low oxygen (9.3 10 -29 bar) and a high sulphur partial pressure (1.2 10 bar). In this environment the sulphidation resistance of various alloys has improved by a factor of at least 100 by the coating. The observed corrosion reaction is local and is explained by a model in which in the first stage cracks are formed due to mechanical stresses in the coating. In the second stage metal sulphides are formed by outward diffusion of metal and inward diffusion of sulphur through the cracks. When stainless steels are used as the alloy the outer layer consists of FeS and the lower one of CrS.