The activity/composition relations of H 2 O-N 2 and H 2 O-CO 2 fluids have been measured at 500 o C, 500 bars. The results are more accurate, and much more precise, than any currently available, especially for H 2 O-poor compositions. Samples were reacted at fixed water activities (0.062 =< a H 2 O =< 0.777), using Cu 3 N as the source of N 2 , and Ag-oxalate as the source of CO 2 . After each experiment the masses of water and gas in the samples were analyzed manometrically. Results depend on the value used for the hydrogen fugacity for pure H 2 O in equilibrium with the oxide buffer, but using a newly measured value for Ni-NiO and fitting the data to a two-parameter Margules equation yields: for H 2 O-N 2 fluids: W G , H 2 O = 3455.0 J/mol, W G , N 2 = 1990.1 J/mol; and for H 2 O-CO 2 fluids: W G , H 2 O = 3882.5 J/mol, W G , C O 2 = 4902.6 J/mol. As uncertainties are not orthogonal, standard errors cannot be given. The data suggest that H 2 O-CO 2 fluids exhibit large positive deviations from ideality at 500 o C, 500 bars, in marked contrast to values predicted by available equations of state. These results indicate that more accurate models for both H 2 O-N 2 and H 2 O-CO 2 fluids are needed.