Factors controlling the solubility of sulfur in mafic magmas are temperature, composition, and fugacities of the gaseous species O2 and S2. In order to determine how these factors control the solubility of sulfur, experiments have been conducted at one atmosphere total pressure and at fO2 and fS2 controlled by mixing of SO2, CO2, and CO. All common mafic magmas are within a range of fO2 and fS2 such that sulfide sulfur is the most important species in the melt. Within this range, a temperature increase of 100 degrees C magnifies the capacity of a magma to dissolve sulfur by a factor of 5 to 7 times, assuming a constant ratio of fO2 to fS2. Increases in fO2 and fS2 cause, respectively, decreases and increases in sulfur solubility. The general relationship between the magma composition and the sulfur-carrying capacity of a magma has also been determined. The strongest positive correlation of sulfur content is observed with the ferrous iron content. Because other components influence the activity coefficient of FeO and thereby the solubility of ferrous iron, their effect on the sulfur-carrying capacity is complex. The maximum solubility of sulfur in a magma of basaltic composition at 1,200 degrees C, was found to vary from 0.05 to 0.2 weight percent as the FeO content was increased from 5 to 20 weight percent. In nature all of the controlling variables change during cooling and crystallization of a magma, and each or all of them may bring a magma to saturation, thereby causing a sulfide phase to precipitate. Under most conditions the sulfide phase which precipitates from a mafic magma will be an immiscible iron-sulfide-oxide liquid.