Industrial experiments on the addition of H2S to the fluid catalytic cracking (FCC) feed (hydrotreated vacuum gas oil) were carried out in the commercial FCC unit at Lukoil Neftochim, Bulgaria. It was found that during cracking of pure hydrotreated vacuum gas oil to which was added about 550 ppm H2S that the FCC gasoline contained about 12 ppm more sulfur than the nonspiked feed. Both gasolines obtained from the pure feed and the H2S-spiked feed contained the same type of sulfur compounds: mercaptans, thiophenes, and benzothiophenes. In the case of the H2S-spiked feed, the additional FCC gasoline sulfur formed was about 1.2% of the sulfur in the H2S added to the feed. The FCC gasoline sulfur formed from the pure FCC feed, on the other hand, was about 2.5% of the sulfur in the feed. The higher percent of sulfur conversion of the pure FCC feed relative to the conversion of the H2S Suggests that both reaction of H2S with olefins or diolefins (from the catalytic cracking of the hydrocarbons of the feed) and trans Formation of heavy sulfur compounds contained in the feed may be responsible for the entire FCC gasoline Sulfur compounds formation.