Derivations of an expression for the generalised outgoing 'adsorption' time-lag pertaining to transport through a slab membrane made by [Rutherford and Do, Review of time tag permeation technique as a method for characterisation of porous media and membranes, Adsorption 3 (1997) 283] and by [Ash et al., Sorption and surface flow in graphitized carbon membranes. II. Time-lag and blind pore character, Proc. Roy. Soc. London, Ser. A, 304 (1968) 407] are not in agreement. Causes for the discrepancy are identified. Two extensions of the generalised procedure are given with particular attention being paid to Delta L, the time-lag difference. (c) 2005 Elsevier B.V. All rights reserved.