Catalyst life in residual oil hydrotreating is dependent on the deposition of coke and metals in the catalyst pores. Although silicon naturally occurs in relatively small amounts in crude oils, recent analysis in this laboratory of used catalyst from a commercial unit showed silicon deposits of one to six percent inside catalyst particles. Laboratory reactor tests simulating commercial operations showed that silicon deposition reduced both catalyst hydrodesulfurization and hydro-demetallization activity. Electron microprobe analysis demonstrated that silicon deposition influenced deposition profiles of metals, sulfur, and carbon.