The data for binary mixtures of volatiles are rather scanty, at least for systems of geological interest, and are of widely differing accuracies. The data are therefore presented in a number of tables with appropriate comments on each so that the uncertainties in the entries may be evaluated. Most of the data have been taken from published work, but not all the data from each source are reported here. In regions of overlap no attempt has been made to arrive at an average value, but instead the data considered to be the better of two sets have been quoted and the other set discarded. Many of the data in the literature are given in non-integral values of composition, pressure, and temperature, and where possible these have been plotted on large-scale graph paper and interpolated to integral values. Where this interpolation is considered to be of the same accuracy as the original data the entries are made with an appropriate number of significant figures, and where the interpolation was over too large a distance or between sets of data due to different authors, or where it was simply extrapolated, the entries are enclosed in parentheses. The data on this type of system fall naturally into two groups: those on the two-phase regions and those on the one-phase regions. Only the supercritical one-phase regions are considered here, and the two-phase regions presented are all of the liquid-vapor type. In the two-phase region the system is described by giving the temperature in° C,