Among the numerous applications of metal-organic frameworks (MOFs), a topical class of nanoporous materials, adsorptive separation is gaining considerable attention. Some of the most exciting candidates for gas separation processes exhibit structural transitions, such as breathing and gate opening. White predictive analytical methods are crucial in separation science and have been widely used for rigid nanoporous solids, a tack exists for materials that exhibit flexibility. We propose here a general method predicting, for the first time, the evolution of structural transitions and selectivity upon adsorption of gas mixtures in flexible nanoporous solids.