Steady, one-dimensional, diffusion in binary ideal gas mixtures is analyzed for both a zero mass average velocity and a zero molar average velocity, to give solutions for equimass and equimolar counterdiffusion. It is shown that the prescription of constant pressure results in equimass counterdiffusion rather than the commonly assumed equimolar counterdiffusion, and that these two situations are fundamentally different. Application to the analysis of venting processes is discussed. (C) 2003 Elsevier Science Ltd. All rights reserved.