Atomic compositions of high quality volcanic gas collections by others from Surtsey, Etna, and Kilauea are examined to determine the compositional range, limits, and trends in the basaltic gas phase. The C-H-S system adequately represents the total composition of these gases: they are restricted to the volume within this tetrahedron bounded by H 2 O, SO 2 , CO 2 , and H 2 . Limiting ratios can be set at C/S greater than 0.500 and C/H less than 1.000. Data on oxygen fugacities of volcanic rocks indicate that the HM (hematite-magnetite) and QFM (quartz-fayalite-magnetite) buffers are extreme limits for terrestrial magmas. The positions of these buffers from 1250 degrees to 800 degrees C within the C-O-H-S tetrahedron plus the limiting C/S and C/H ratios indicated by collections limit the compositional range over which bulk gas compositions compatible with magmas can vary. The distance between the buffers in the tetrahedron increases greatly with increased carbon and sulfur and decreased temperature, allowing a much wider range of gas compositions. If magmas are buffered by their gases, only a restricted range of compositions near HM would be possible. When magma buffers the gas, the greatest compositional range (defined by HM, 1250 degrees C and QFM, 800 degrees C) is possible. The wide range of collected gas compositions indicates that gases are commonly buffered by magma. Further, gas compositions are apparently more closely related to QFM buffering than to the oxidized HM extreme. The general correlation of collected gas compositions with QFM buffer positions at the collection temperatures strongly supports these conclusions. The increase of C/S with decreased temperature suggested by the collection data is also in agreement with the evolutionary trends shown by the above volcanoes. Plots of C/S versus C/H for the best samples from each of the three volcanoes show a compositional trend that is best explained by late stage magma degassing. This trend indicates hydrogen enrichment and sulfur depletion, gases obtained by basalt degassing plot at the hydrogen-rich, sulfur-poor end of this trend. This common trend may indicate a similar compositional evolution during magma degassing for the three volcanoes. Alternatively, differences in initial bulk composition at the source may explain the separation of the volcanoes on this trend.