The partitioning of copper has been investigated experimentally between a natural pyrrhotite and a natural high silica rhyolitic melt under water-saturated conditions at 800 degrees C, 1 kbar, and two oxygen fugacities. The partition coefficients, D (super Po/melt) Cu , defined as the ratio of the concentrations of copper in pyrrhotite to copper in melt, are 910 + or - 436 (1Sigma ) at the C-CH 4 buffer and 554 + or - 220 (1Sigma ) at one-half of a log unit above NNO. The mean value for all determinations is 773 + or - 400 (1Sigma ). Although the experimental results show wide scatter, at test indicates that the partition coefficients at the two oxygen fugacities may be considered significantly different at the 88 percent confidence level. The results are consistent with hypothetical chemical potential equilibria and with previous field and experimental data. Estimates obtained through mathematical modeling suggest that such partition coefficients may result in sequestration of 98 percent of the copper by pyrrhotite in a relatively S-rich and H 2 O-poor magma, whereas 24 percent of the copper may be sequestered by pyrrhotite in a relatively low sulfur magma in which water saturation and evolution of an aqueous fluid occur early. The results may provide significant constraints in interpretation of the behavior of Cu in felsic magmatic systems and in application of the orthomagmatic hydrothermal hypothesis to geologic processes associated with porphyry-type copper deposits.