From inspection of the acetonitrile free energy curve, several inflection points between the two stable phases were observed (Figure ). The free energy is equal to the fluid surface excess energy, meaning that inflections are indicative of mesophase formation -analogous to bubbles in a bulk 180 fluid. Further, as this free energy is associated with the formation of an interface, mesophase formation indicates that the energy is not evenly distributed within the MOF -instead the energy is concentrated at specific points within the material. To confirm the presence of acetonitrile mesophases within IRMOF-1 (and the lack of any similar mesophases for dichloromethane), we directly analysed the solvent configurations in the framework during each simulation. By plotting the 185 sorbate probability density distribution within the framework at densities of 330-410 kg/m 3 and 200-240 kg/m 3 for dichloromethane and acetonitrile, respectively, mesophase formation can be observed directly (Figure ). These sorbate densities correspond to the free energy maxima in Figure , representing the transition state between the two stable phases. In agreement with the free energy profiles, dichloromethane was distributed evenly throughout the framework, with dense clusters 190 surrounding each Zn4O secondary building unit and evenly occupying each pore within the framework (Figure ). In contrast, acetonitrile is preferentially located in the larger of the two pores in the MOF (Figure ), forming an interface internally within the framework. Note that simulation in a supercell containing 2x2x2 unit cells showed the same evenly distributed mesophases demonstrating that the behaviour can be adequately described in the smaller system (Animated Figure ). 195 Phase coexistence within MOF pore spaces has been previously observed in theoretical and 200 experimental studies of benzene adsorption in IRMOF-1. There, the bimodal pore size of IRMOF-1caused by the orientation of the benzene rings in the linker either pointing into or out of the resulting pores -was highlighted. Benzene was shown to preferentially adsorb into the larger of the two pores with a significant energy barrier to move between the pore spaces. These findings are also in agreement with previous studies showing that phase coexistence within MOFs can exist for any 205 sufficiently subcritical fluid e.g. methane at 50 K, suggesting that the behaviour can be controllably switched by varying the temperature and pressure.