An apparatus for vapor pressure measurement with a very small cell by the mass-loss Knudsen effusion technique was tested with solid benzoic acid and anthracene. The vapor pressure and enthalpy of sublimation results of the two reference compounds were in good agreement with accepted literature data. The vapor pressures at different temperatures of 17 polychlorinated dibenzo-p-dioxins (including dibenzo-p-dioxin) and five polychlorinated dibenzofurans (including dibenzofuran) were measured with the apparatus, and the enthalpies of sublimation of the 22 dioxins and furans were derived from the temperature dependence of vapor pressure. The results were systematically compared with the literature data.