Pyrolysis of used sunflower oil was carried out in the presence of different amounts of HZSM-5 at 400 and 420 degrees C in a reactor equipped with a fractionating packed column, the length of which was varied. The products consisted of gaseous and liquid hydrocarbons, acids, CO, CO2, water, and coke. The compositions of the gaseous and liquid products were studied by gas chromatography. The product yields and compositions were affected by catalyst content, temperature, and column length. Nearly complete conversion (96.6%) of the used oil and the maximum liquid hydrocarbon yield (33%) were obtained at the highest temperature (420 degrees C), highest catalyst content (20%), and the lowest column length (180 mm) employed. The aromatic hydrocarbon contents of the liquid hydrocarbon products, which consisted of hydrocarbons of gasoline range, were in general lower than those obtained using fixed bed reactors but increased parallel to the increase in catalyst content. Additional thermal reactions taking place in this type of reactor increased with the increase in column length. Although the total conversion and total liquid hydrocarbon yields were influenced adversely by these reactions, liquid products with higher isomeric hydrocarbon contents were obtained with longer columns.