The vapour-phase reaction of crotonaldehyde and carbon disulfide was performed in a fixed-bed flow reactor in the presence of promoted chromia on gamma-alumina catalysts, namely G41 and GI IP from Girdler, and Cr-0211T from Harshaw. Although more than 30 recation products or intermediates were identified, three principal pathways were involved: thiophene, olefin, and tar. The alkali-promoted G41P catalyst was shown to exhibit the best performances. At 440 degrees C, with a contact time of 5.8 s and a flow rate of 0.03 ml/min, the selectivity to thiophene reaches about 70% up to, at least, 20 h on stream without catalyst de-activation. On the other hand, addition of water to the reaction mixture led to a slight increase in the selectivity to thiophene as well as in the formation of benzene. Although several mechanisms were proposed for this reaction, it seemed that the presence, even in low amount, of dihydrofurane and dihydrothiophene species cannot be ignored as far as those intermediates are also involved in the reverse reactions, HDO of furane and HDS of thiophene. According to the microreversibility principle, cyclization of crotonaldehyde into 2,3-dihydrofurane was proposed as the first reaction step, followed by sulfidation and de-hydrogenation steps.