Recent work from our laboratory has focused on chemical warfare agents CWAs, simulants, precursors, and degradation products. This report documents the vapor pressure data for three organophosphonate ester compounds and three phosphonic acids, all of which are related to CWAs. Due to structural similarities, these materials may also be considered to be CWA simulants and can be used to evaluate the performance of defensive equipment, which requires accurate knowledge of the thermophysical properties of these materials. Vapor pressure data were measured using two ASTM International methods that are based on differential scanning calorimetry DSC and vapor saturation. The data presented herein have been used to determine temperature-pressure correlations, and the fit constants have been used to determine temperature-dependent thermodynamic properties and to enable interpolation and limited extrapolation of the data. The criteria for assessing vapor pressure data quality are presented herein.