A low sulfur vacuum gas oil (VGO) has been spiked with sulfur containing compounds belonging to the gasoline and light diesel range, and the feedstocks obtained in this way have been cracked over an equilibrated catalyst in an automated MAT unit. The evolution of these sulfur compounds under catalytic cracking conditions has been studied using a specific sulfur detector. The reactivity of the sulfur compounds as well as their interaction with olefins and naphthenic compounds has been elucidated.