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.