Isosteric heats of chemisorption of sulfur on iron and cobalt have been evaluated from measurements of the equilibrium ratio of H2S/H2 gas mixtures in contact with the powdered metals over a temperature range from 677 to 855 K. At low surface coverage of sulfur on cobalt, the heat of chemisorption with respect to 1/2 S2(g) is -236±15 kJ mol-1. With increasing coverage it decreases to -200±10 kJ mol-1. The heat of sulfur chemisorption on iron is -197±15 kJ mol-1. For equivalent coverage, the thermodynamic activity of sulfur chemisorbed on cobalt exceeds that of iron by approximately one order of magnitude. At low surface coverage (<0.5 saturation coverage), the bond strength of sulfur on the various metals is found to vary in the order: Ni≳Co≳Ru≳Fe.