Oil/saline partition coefficients for inhaled compounds often are defined by the ratio of the separately determined oil/gas and saline/gas partition coefficients. This approach assumes that the concurrent presence of oil with saline has no effect on the characteristics of either solvent. To test this assumption, we measured the oil/gas and saline/gas partition coefficients for CF3(CCIF)2CF3 and 1,2-dichloroperfluorocyclobutane (C4Cl2F6) separately and with the two phases mixed in a common container. We chose these compounds because they have radically different oil/gas and saline/gas partition coefficients and thus would provide a severe test of the assumption. For CF3(CCIF)2CF3, olive oil/saline partition coefficients were, respectively, 13,200 and 13,300 when measured separately and in mixed phases, and the octanol/saline partition coefficients were 19,200 and 18,100. Similarly, olive oil/saline partition coefficients for 1,2-dichloroperfluorocyclobutane were 3660 and 3500 when measured separately and in mixed phases, respectively, and the octanol/saline partition coefficients were 5140 and 4560. We conclude that differences between separate and mixed-phase determinations of ratios are small or nonexistent.