An apparatus for very low vapor or sublimation pressure measurements (10(-3) Pa) was designed and built. The principle of the apparatus is based on the saturation by the sample and by a standard compound of an inert gas flowing through a saturator and into a cold GC column used as a trap. This stage is followed by a chromatographic analysis consisting of increasing the temperature of the column. The originality of the present apparatus lies on the use of a supplementary line containing the standard compound. The study of the two substances is carried out simultaneously. This relative method allows reliable measurements of vapor or sublimation pressures. The apparatus was tested by the study of several alkanes. The experimental pressures are in a good agreement with the available literature data.