Since careful consideration must be given to the catalyst used in catalytic cracking operation in the petroleum industry to assure economic operation, laboratory catalyst activity test units have been developed. These units approximate the conditions in large scale commercial cracking units. It was the purpose of this investigation to determine the amount of cracking that was obtained from catalytic effects in cracking a standard light East Texas gas oil over a Houdry pellet catalyst when compared to the cracking over silica at temperatures from 500 to 1200 °F in a catalyst activity test unit. A catalyst activity test unit was used to determine the percentage conversion of the feed oil to lower molecular weight hydro-carbons using a Houdry pellet catalyst in one series of tests and silica, which is regarded to be noncatalytic, in another series. Fifteen determinations were made at various temperatures from 580 to 1190 °F and a space velocity of 1.0 volume of feed per volume of catalyst per hour. Ten determinations were made at a space velocity of 2.0 volumes of feed per volume of catalyst per hour at temperatures from 595 to 1160 °F. Data were collected on the quantity of liquid and gaseous products produced, as well as the operating conditions employed. After each cracking determination, the packing was regenerated by heating in the presence of air to burn off any carbonaceous deposits. At a space velocity of 1.0. and various temperatures ranging from 580 to 1190 °F the use of Houdry pellet catalyst produced more cracking than silica at like temperatures. The use of the catalyst effectively reduced the temperatures of the cracking reactions approximately 300 °F at a space velocity of 1.0. The effect of the catalyst was lessened by the effect of temperature at approximately 1200 °F and a space velocity of 1.0. When cracking over silica changing the space velocity from 1.0 to 2.0 raised the temperature required 50 °F for a given amount of cracking. / Master of Science