A closed-loop gas-recirculation system was used to measure the isosteric heat of sulfur chemisorption on supported nickel catalysts in hot gas cleaning conditions of gasification gas. During sulfur adsorption, reconstruction of the catalysts occurred. In addition, probably the enormous increase in surface diffusion due to sulfur adsorption on some nickel catalysts with high flow rates resulted in melt formation of adsorbed species on the surfaces of catalyst particles. Heat of sulfur adsorption on nickel decreased when sulfur coverage was increased. However, the enthalpy of adsorption decreased even below the heat of formation of bulk Ni3S2, indicating most likely multi-layer or subsurface sulfur formation on catalyst surfaces. The structural properties of the catalysts had a great influence on sulfur adsorption behavior. The effect of sulfur on ammonia decomposition in synthetic gasification gas tests was explained by the change of heat of sulfur chemisorption on nickel.