Gas flow through crimped-capillary leaks was studied with the use of a mass spectrometric system. The crimped-capillary leak was used as a single-stage pressure-reduction device between a gas inlet manifold and an ultrahigh-vacuum mass spectrometer. The steady state flows of binary gas mixtures (H-2-He-4 and H-2-D-2) through the crimped-capillary leak into the mass spectrometer vacuum chamber (<10(-9) Torr) were characterized over a pressure range from 5 to 5000 Torr. At pressures <5 and >1500 Torr, the gas flow is close to Knudsen diffusion (molecular flow) and viscous flow, respectively. The transition from the Knudsen diffusion to viscous flow takes place over 2 decades of pressure and is at a lower pressure range than can be explained by the combined Knudsen diffusion and viscous flow theories. This article considers the contribution of molecular diffusion (ordinary diffusion) to the theory of gas flow through crimped-capillary leak and presents a mathematical model for a multicomponent gas mixture. This model successfully predicts gas flow over a wide range of pressures especially in the transition flow regime for binary gas mixtures. (C) 2003 American Vacuum Society.