Abstract Experimentally determined oxygen fugacities in equilibrium with the original ferrous to ferric iron ratios of nine basalts and andesites range from 10 −8·5 to 10 −6·4 atm at 1200°C. There is a strong tendency for the ƒ O 2 to increase with increasing acidity of the rocks studied. Theoretical calculations of equilibrium ƒ O 2 's, using equilibrium constants determined from single experimental runs, gave results reasonably close to the actual values provided runs in the ƒ O 2 range 10 −6 to 10 −8 atmospheres were used. Calculations based on runs made in an air atmosphere were in substantial error. Based on the determined ƒ O 2 's of four rocks from the Mt. Hood area, a probable maximum range in ƒ O 2 of 10 −9 to 10 −6.5 atm is deduced for rocks of the Cascade Province, ranging from most basic to most acidic, at their liquidus temperatures. If the oxygen fugacity is important in controlling the differentiation trend of Cascade rocks, the mechanism must be operative within this range.