Abstract The flow of gases in the transition zone between laminar ( Poiseuille) and molecular ( Knudsen) flow is of great importance in vacuum technique is finding increasing application in modern chemical industry. Moreover, there are special separation effects in this transition zone 1 1 H.-D. Beckey u. W.E. Groth, Naturwiss. 1951 38 558. Z. Naturforschg. 1952 7a 474, entdeckten im Übergangsgebiet an Kapillaren, die sich in einem Temperaturgefälle befanden, einen bisher unbekannten Trenneffekt für Gasgemische, bein dem sich die leichte Komponente am kalten, die schwere am warmen Kapillarenende anreichert (“negativer Effekt”). Im Bereich der reinen Knudsen-Strömung konnte der Effekt zu einem “positiven umgekehrt werden. . Our knowledge of the flow through micropores, which often takes place even at atmospheric pressure in this transition zone is lacking. This investigation is concerned with the problem mentioned above. A short discussion is devoted to the transitory flow through single capillaries, while the flow minimum involved is qualitatively explained. The different behaviour of the flow through micropores is attributed to the influence of the pore spectrum, and a new method of determining the “mean pore radius” is given. The effect of the mean gas pressure and of the temperature on this flow is examined, and the sometimes great influence of the slip along the walls and the dependence of the mean free path of the gas molecules on temperature is elaborated. Finally, the conditions are defined under which surface diffusion may come to play an important part in the direct flow through micropores.