The vapor pressure of hexachlorobenzene (HCB), measured with the gas saturation technique, was related to the temperature by the equation log₁₀p = 12.94 − (5279/T) giving HCB vapor pressure values from 0.420 × 10⁻⁵ to 22.4 × 10⁻⁵ mm Hg over the temperature range 15 to 45° C. The heat of vaporization of HCB was calculated to be 24.2 kcal/mol. The vapor pressure data was used to measure the vapor phase diffusion of HCB in soil using a vapor-diffusion cell. The steady-state vapor diffusion of HCB in soil was directly related to soil air-filled porosity and was therefore greatly reduced by increased soil bulk density and increased soil water content. The apparent steady-state HCB diffusion coefficient was linearly related to the soil porosity term, Pₐ¹⁰/³ / PT² where Pₐ is the soil air-filled porosity and PT is the total soil porosity. The calculated diffusion coefficient in air for HCB was 1.0 × 10⁴ cm²/day.