Following Favre et al. [J. Membr. Sci. 207 (2002) 59], a detailed study has been made of the effect of time-dependence in the boundary conditions upon the four measurable time-lags associated with transport through a membrane. A new procedure for deriving the modified time-lags is presented which encompasses three membrane geometries and a variety of boundary conditions. While the greater part of the treatment involves a constant diffusion coefficient, some attention has been given to the corresponding problem of transport in a slab membrane,governed by a concentration-dependent diffusion coefficient. The application of this procedure to a particular aspect of constant-pressure sorption kinetics is also indicated. (C) 2003 Elsevier B.V. All rights reserved.