The phase behavior of natural gases containing elemental sulfur has been modeled using a chemical equilibrium model and the Peng-Robinson equation of state. Sulfur in the fluid phases is modeled as a mixture of eight species, S-1 through S-8, and reactions of each with hydrogen sulfide to form eight different sulfane species are anticipated. A proposal is made for constructing the Peng-Robinson parameters for all species from the parameters for source substances including S-8 and H2S and interaction parameters only between the source substances. A separate model is used for sulfur as a solid. Comparisons are made with data for pure sulfur and for sulfur in contact with a number of gas mixtures containing hydrogen sulfide, carbon dioxide, methane, and nitrogen. The equilibrium sulfur content of the gases is well predicted over a wide range of gas composition, temperature, and pressure.