An experimental technique is described in which both the kinetics of nucleation of vapor deposits and certain of the related physical processes, such as the thermal accommodation of vapor at the substrate surface and the adsorptive properties of the nucleant species, can be measured simultaneously. The technique described is based on the use of a small mass spectrometer to monitor the flux of vapor molecules leaving the substrate while adsorption or nucleation processes are taking place. Results obtained for cadmium vapor impinging on an atomically clean tungsten surface indicate the formation of two distinct adsorbed phases. A primary phase, obeying the relation τa=0.94×10−10exp(41100/RT), forms on initial exposure of the clean substrate to the vapor. At lower temperatures a secondary phase, which obeys the relation τa=1.42×10−10exp(21200/RT), forms on top of the primary phase. Measurements of the critical supersaturation required for crystal nucleation indicate. that adsorbed layers, several monolayers thick, are present on the surface prior to crystal nucleation. Thus the nucleation process in this case cannot be described in terms of current theories.