While the use of selective additives and catalysts might be the most economical and easiest solution for refinery operations in reducing the sulfur concentration in catalytic cracked naphtha their effectiveness is still limited. A better knowledge on the chemistry of sulfur compounds under cracking conditions would help to improve the performance of such additives. In this work, the sulfur content (in terms of alkyl-thiophenes and benzothiophene) in cracked naphtha was studied in regard to the feed and catalyst properties and the effect of a commercial sulfur reduction additive. Two equilibrium catalysts (REHY and REUSY) and two feedstock from Isthmus and Maya crude oils were compared in a fluidized fixed bed microreactor. The sulfur content in the naphtha was lower with REHY thus confirming reported trends involving hydrogen transfer and contaminant metals effects. The sulfur compounds contained in the Maya feed were comparatively more reactive than those contained in a conventional Isthmus/Maya (70/30). Assuming that the Maya feed is richer in long chain alkyl-thiophenes it produced proportionally higher amount of C-1-C-4 alkyl-thiophenes in the naphtha. The additive reduced the amount of C-1-C-4 alkyl-thiophenes by preventing their formation and producing alkyl-benzothiophenes by hydrocyclization. REHY produced less sulfur-in-naphtha by converting such compounds to H2S and light gases. The additive showed a limited effect on naphtha sulfur reduction with either the Maya feed or REHY An inhibitory effect due to H2S adsorption is also proposed to explain this limited effect.