The reaction of FeS+ with D-2 is examined by guided-ion beam mass spectrometry. Three products, Fe+, FeD+, and FeSD+, are formed in endothermic processes, and thresholds for these reactions are determined. Comparison of the thresholds with literature thermochemistry reveals considerable activation barriers in excess of the endothermicities for the formation of Fe+ and FeSD+. Additional bracketing and equilibrium measurements in a Fourier transform ion-cyclotron resonance mass spectrometer yield Do(Fe+-SH) = 66.0 +/- 2.6 kcal/mol. A potential-energy surface of the system [Fe,Hz,S](+) predicted by density functional theory is used to interpret the experimental data. According to these calculations, the lowest-energy path for Fe-S bond activation involves 1,2-addition of hydrogen across the Fe-S bond along with spin inversion from the sextet to the quartet surface.