Abstract A series of fixed-bed nickel hydrogenation catalysts was tested in the dearomatization of hydrocarbon solvents. The mechanism of catalyst deactivation by aromatic sulfur compounds was studied in high-pressure micro-flow equipment by variation of the experimental conditions and the sulfur content of the feed. It is concluded that catalyst deactivation proceeds under mild but realistic conditions through formation of a surface sulfide which blocks the active surface. The rate of the disappearance of the active sites is a first-order process with rate constant 1.0 × 10 −3 (ppm S) −1 h −1. Under more severe conditions, more sulfide layers are formed, but bulk Ni 3S 2 was not observed even after full deactivation of the catalysts. The poisoning of the active sites in the latter case is no longer a first-order process. Consequently, under the circumstances investigated, the sulfur resistance of nickel catalysts is determined by the nickel surface area per unit weight of catalyst.