Two possibilities were considered to account for the formation of sulfur impurities in fluid catalytic cracking (FCC) gasoline: the cracking of long alkyl chain thiophenes, and the addition of H2S to olefins or diolefins followed by cyclization. The transformation of compounds representative of the sulfur components of FCC gasoline or of their possible precursors (thiophene, tetrahydrothiophene, 2- and 3-methylthiophenes, 2-ethylthiophene, 2- and 3-n-hexylthiophenes) as well as the transformation of hex-1-ene, penta-1,3-diene and hexa-1,5-diene in the presence of H2S were studied over a commercial FCC catalyst at 500degreesC under atmospheric pressure. Tetrahydrothiophene, methylthiophenes, 2-ethylthiophene and the n-hexylthiophenes were found quite reactive under conditions similar to those of FCC, whereas thiophene was not. Desulfurization was not observed with methylthiophenes and 2-ethylthiophene, while this reaction occurred to a significant extent with tetrahydrothiophene and also with n-hexylthiophenes, which shows that the length of the alkyl chain has a great influence on the reactivity of alkylthiophenes. The formation of alkylthiophenes from H2S and olefins or diolefins was observed, although the yield was very low. Reaction mechanisms are proposed to explain all these reactions. It is concluded that the two ways which were considered can lead to the sulfur-containing compounds present in FCC gasoline.