The process of catalytic cracking of petroleum hydrocarbons invariably produces a carbonaceous deposit on the surface of the solid catalyst which serves to reduce the activity of the catalyst. This investigation was undertaken to determine the effect of the feed rate and the length of the cracking period on the carbon formation on the catalyst. A vertical, fixed-bed, externally heated reactor was constructed. The reactor contained a bed of synthetic silica-alumina Socony-Vacuum bead catalyst. The necessary auxiliary apparatus required to handle the feed and the products of reaction was provided. The amount of carbon deposited on the catalyst was determined by burning it and measuring the carbon dioxide thus formed. The feed material used was Esso Diesel Oil (208). It was exposed to the catalyst at a temperature of about 900 degrees Fahrenheit for a series of ten minute periods while the feed rate was varied from 0.42 to 5.55 volumes of feed per volume of catalyst per hour. Another series of tests was performed at the same temperature, the feed rate being held at approximately two volumes of feed per volume of catalyst per hour; while the length of the cracking period was varied between thirty-five seconds and thirty minutes. It was found that during the ten minute cracking periods the amount of carbon deposited on the catalyst was independent of the feed rate between the limits of 0.91 and 5.55 volumes of feed per volume of catalyst deposited on the catalyst was related to the length of the cracking period by a parabolic function. Further analysis of the data revealed that the amount of feed converted into carbon was related to the degree of conversion by a parabolic function. From the three relations mentioned above, an equation relating the conversion, the feed rate and the cracking period was derived. The form of this equation is as follows: V³⋅⁰ = (9.60 x 10⁵)/R_fv t⁰⋅⁷⁵ where V is the percent conversion, R_fv is the feed rate and t is the cracking time in minutes. It was further found that the activity of the catalyst was not materially decreased after a series of twenty-nine tests, but that the substitution of quartz chips for the catalyst in the reactor decreased the conversion obtained by 75 per cent. / M.S.