The interaction of magma-gas systems is investigated, employing fugacity data derived from mineral equilibria in basaltic lavas. These data are compared with equilibrium compositions in the C-O-H-S gaseous system to identify atomic and molecular compositional characteristics of gases compatible with lavas over a range of physical conditions. C-O-H-S gases with equilibrium oxygen fugacities (fO2) on NNO or QFM at 1000 degrees C and 1 atm total pressure will have fO2's during closed gaseous system cooling which either continue to be compatible with these buffers or which become too high for such equilibria. The degree of compatibility depends upon the bulk atomic composition of the gas. Similar results are obtained at moderate pressures. Oxygen fugacities measured in lavas or determined from rock mineralogies also follow these trends with decreasing temperature, and a wide range of bulk gas compositions are compatible with these lavas. However, the trend toward increasing reduction with decreasing temperature at Skaergaard is not followed by any gas during closed system cooling. Others have shown that subaerial lavas are commonly more reduced than submarine lavas. This may be caused primarily by the fO2 decrease in gases during pressure decrease, even at constant composition, rather than by the effervescence of SO2 during magma degassing. The effect of the latter process is relatively minor. The atomic compositions of gases compatible with pyrrhotite-magnetite assemblages crystallized from immiscible sulfide liquids at Alae Crater, Hawaii have been determined. at 800 degrees C, 1 atm total pressure, the allowed range of gas compositions for these assemblages includes abundant carbon and sulfur and excludes compositions dominated by H2O. SO2 is more abundant than H2S, and, at C/S = 1.5, the molecular composition is like that derived for the "initial" gas at Kilauea (Nordlie, 1971), At 900 degrees C, 1 atm total pressure, only gases with anomalously low C/S ratios are allowed. The allowed range of gases compatible with oxide-sulfide assemblages at higher pressures includes more water rich compositions. However, data from submarine lavas indicate that such gases may not be favored at higher pressures. Pyrrhotite-magnetite assemblages and immiscible sulfide liquid are favored in environments allowing rapid quenching or maintenance of early magmatic gas compositions by confining pressure.