The equilibrium properties of the H2O-HNO3-N2O5 system is represented using the MSA electrolyte activity model developed by Ball et al [1]. As our purpose is to obtain a good representation of both VLE data and the liquid phase composition, the data base used to adjust the parameters includes VLE experimental values and also data relative to real species concentrations, deduced from Raman spectroscopy measurements, some of them being obtained during the present study. The quality of the representation compares well with the experimental deviations.