The catalysts used in the fluid catalytic cracking process are reversibly deactivated by deposition of coke. The coke deposition and its effect on the activity of the catalyst in the reactor have been studied using the microriser, a laboratory-scale entrained flow reactor. The experimental results show that the timescale of coke formation is much shorter than the timescale for conversion. Coke deposition mainly occurs initially (within 0.15 a), while the conversion increases during the whole residence time in the reactor (0-5 s). A five lump kinetic scheme has been used to model the measurements with a constant catalyst activity and with a catalyst activity that decreased with time on stream. It is shown that the results obtained after 0.15 a can be modeled with a constant activity, so initial coke deposition is the main cause of deactivation. It is proposed that the initial effects, coke deposition accompanied by formation of products, and catalyst deactivation, have to be described with a separate model that takes into account catalyst-to-oil ratio, feedstock, and catalyst properties.