Phosgene and nitrogen and, moreover, chlorine, carbon monoxide, and nitrogen gas mixtures were used for the chlorination of aluminum oxide in the temperature range 450-850-degrees-C. A mechanism and a mathematical model were developed to describe processes playing various roles during chlorination. The mathematical model is based on previous suggestions and on experimental observations. It was assumed that the aluminum oxide particles are surrounded by a mixture of chlorine, carbon monoxide, phosgene, and nitrogen gases, and the concentration of gas components does not necessarily correspond to the equilibrium of the COCl2 half-arrow-right-over-half-arrow-left CO + Cl2 reaction.