The magmatic oxygen fugacity (fO2) of a thirty foot wide feldspathic peridotite dyke has been determined using the experimental method of Fudali (1965). Determinations were made on samples from both the marginal and central portions of the dyke and a difference of approximately one order of magnitude in fO2 was observed. This difference is attributed to the increase in the H2O content of the liquid as crystallization proceeded and to diffusion of hydrogen out of the dyke. It is concluded that the dyke was emplaced with an fO2 between 10−8 and 10−9 atmospheres. Data on the absorbtion of Fe by the silver/palladium sample containers during the experiments are given in an appendix.