Phase behavior of confined fluids adsorbed in na-nopores differs significantly from their bulk coun-terparts and depends on the chemical and structural properties of the confining structures. In general, phase transitions in nanoconfined fluids are reflect-ed in stepwise adsorption isotherms with a pro-nounced hysteresis. Here, we show experimental evidence and in silico interpretation of the reversi-ble stepwise adsorption isotherm which is observed when methane is adsorbed in the rigid, crystalline metal-organic framework IRMOF-1 (MOF-5). In a very narrow range of pressures, the adsorbed fluid undergoes a structural and highly cooperative re-construction and transition between low-density and high-density nanophases, as a result of the competition between the fluid-framework and flu-id-fluid interactions. This mechanism evolves with temperature: below 110 K a reversible stepwise iso-therm is observed, which is a result of the bimodal distribution of the coexisting nanophases. This temperature may be considered as a critical temper-ature of methane confined to nanopores of IRMOF-1. Above 110 K, as the entropy contribution in-creases, the isotherm shape transforms to a common continuous S-shaped form that is characteristic to a gradual densification of the adsorbed phase as the pressure increases.