The residual asphalts obtained from three Nigerian medium crudes were chemically treated with elemental sulfur at 210 to 250°C. The course of the reaction was followed by monitoring the growth of the asphaltene content of the products. The sulfurization reaction led to 5- to 10-fold increases in the asphaltene content of the products. The asphaltene content of one of the samples initially increased with temperature from 210 to 240°C and then dropped sharply. The results obtained revealed that the optimum temperature of the sulfurization reaction was 240°C. It was also observed that the lower the asphaltene content of the untreated asphalt, the higher the growth of the asphaltene content of the product at the same reaction temperature. The absolute viscosity of the sulfurized asphalts increased rapidly with increasing temperature of reaction.