A procedure has been developed at the National Bureau of Standards utilizing isotope dilution spark source mass spectrometry (ID-SSMS) for the determination of sulfur in iron-base alloys. With this technique a known amount of highly enriched sulfur-34 (³⁴S) isotope (spike) is added to the sample, and, after physical and chemical equilibration between the spike sulfur and the natural sulfur in the sample, the equilibrated sulfur is isolated by reduction to hydrogen sulfide (H₂S) and precipitation as silver sulfide (Ag₂S). The altered isotopic ratio of the sulfur (³²S/³⁴S) is then measured with the spark source mass spectrometer. The sulfur concentrations are calculated from the sample weight, spike weight, measured altered isotope ratio, and known isotopic abundance of ³⁴S and ³²S in both the natural and spike sulfur. The key step in obtaining a quantitative sulfur analysis is the dissolution of the sulfur-containing iron samples in a sealed tube. This dissolution procedure enables the sample sulfur and the spike sulfur to equilibrate completely without any possibility of loss of either species by volatilization.