We present here a molecular simulation study of the adsorption of CO2 in three IRMOFs, a family of metal−organic frameworks with varying pore size and chemical nature, as controlled by the linker length and the strength of the MOF−CO2 interactions. This extension of previous theoretical and experimental characterizations (Walton, K. S.; Millward, A. R.; Dubbeldam, D.; Frost, H.; Low, J. J.; Yaghi, O. M.; Snurr, R. Q. J. Am. Chem. Soc. 2008, 130, 406−407) provides a coherent explanation and a generic framework in which to understand the presence or absence of step in the sorption isotherms in terms of the effect of confinement on the liquid−gas transition in the phase diagram of the adsorbate. This branch of the phase diagram is calculated explicitly for CO2@IRMOFs and is compared to previous work on families of zeolites and other metal−organic frameworks.