In this paper, we examine two different formulations of the Kremser equation for sizing mass exchange units. These formulations overcome the inherent singularity in the Kremser equation when the absorption factor A is equal to unity, by expressing the terms in the equation in a form which reduces to a ratio of two logarithmic mean values. These logarithmic mean values are then evaluated using the Underwood approach, yielding elegant expressions which give answers within 1% of the true values except at extreme values of the system parameters. It is proposed that these expressions should be adopted for sizing mass exchangers, especially in mixed-integer non-linear programming (MINLP) solutions for mass exchange networks. (C) 2004 Elsevier Ltd. All rights reserved.