The equilibrium conditions in the system above on were obtained by a differential vapor pressure method and the results up to 200 degrees are reported. From measurements of the differences in vapor pressure between pure water and a salt solution it is possible in principle to obtain activity coefficients for the ions in solution. The vapor pressures of sodium chloride and sodium carbonate solutions up to 200 degrees are given, but it was not possible to obtain results of sufficient accuracy to derive activity coefficients.