The fouling characteristics of a coker gas oil (CGO) containing measurable amounts of olefins and conjugated olefins were investigated using a bench-scale hot liquid process simulator and a batch autoclave reactor at different temperatures. At low surface temperatures (similar to 200 degrees C), the fouling propensity of CGO is very low, and at surface temperatures between similar to 250 and 325 degrees C, the fouling propensity is slightly higher because of polymerization of unsaturated olefins. When the surface temperature is further increased to above 350 degrees C, fouling is observed to increase significantly because of familiar coking reactions. Slow polymerization of olefins and conjugated olefins was observed at temperatures of 270 and 300 degrees C. However, at 350 degrees C, thermal cracking reactions that produce olefins and conjugated olefins take place and significant amounts of fouling deposits can be formed at this temperature. The fouling deposits of CGO exhibit typical polyaromatic coke structures. A better understanding of how the various mechanisms interact with temperature will improve management of fouling in process streams containing olefins and conjugated olefins.