The vapor pressure of the vesicant chemical warfare agent, bis(2-chloroethyl) sulfide ( also known as sulfur mustard or HD), has been measured at T = (-25 to +20) degrees C using a modified ASTM vapor saturation methodology. This work represents the initial report of the measured vapor pressure of HD below its melting point, +14.45 degrees C. The data measured for solid-phase HD are consistent with the previously reported enthalpy of fusion and liquid-phase vapor pressure of HD.