A flow recirculation, isotopic tracer method was employed to determine sulfur uptake and sulfur exchange for a number of catalysts consisting of combinations of Mo, W, Ni, Co, Pd, and Pt, supported on alumina. By considering adsorption and exchange equilibria, basic equations were derived for calculation of extent of sulfur heteroexchange and fraction of exchangeable sulfur in the sulfided catalysts. Exchange equilibrium was experimentally established by approaching exchange from opposite directions. Variations in sulfur uptake and extent of exchange were obtained for the different catalysts. In all cases, the exchangeable sulfur was less than the total sulfur content. It is proposed that the fraction exchangeable sulfur is related to the edge S to total S ratio of the basic MoS2 particles. On this basis, average MoS2 slab sizes were estimated by application of slab models. The amount of exchangeable sulfur correlated reasonably well with thiophene hydrodesulfurization activity of the catalysts.