A corresponding states type model has been used to estimate the second and third virial coefficients of the NH3 + H2O mixture. Additional mixing parameters were added to the formulations for both coefficients to account for the chemical interactions between unlike molecules. The mixing parameters were optimized using experimental gas density data from 373 to 498 K and phase equilibrium data at temperatures below 400 K, and the model (in conjunction with a suitable activity coefficient model) represented these data satisfactorily. Extrapolation of the model to higher temperatures and pressures revealed good agreement with existing VLE data to 589 K and with new density data to 773 K at pressures over 10 MPa.