The assumption of linear equilibrium in ZLC experiments is investigated to evaluate possible errors in the determination of diffusional time constants in microporous materials. The shape of the response curves is obtained considering the Langmuir isotherm which adds an additional parameter (nonlinearity) to the modelling equations. The analytical solution for the limiting behaviour under equilibrium control conditions as well as a closed-form analytical approximation for long time asymptotic behaviour are derived. It is shown that equilibrium controlled systems yield response curves which are qualitatively similar to those obtained under conditions of kinetic control. In order to exclude the intrusion of nonlinear equilibrium control effect it is necessary to consider simultaneously experimental response curves at different flow rates. Integral response curves are also discussed with reference to experimental results available in the literature.