We present here compelling evidence for adsorbate restructuring as the microscopic origin of hysteresis in closed end pores. Our argument is based on molecular simulations in the grand canonical (GCE) and meso-canonical (MCE) ensembles, for argon adsorbed at 87 K in slit mesopores of different topologies (open end, closed end and closed pores). The MCE isotherms have sigmoidal van der Waals type loops, while the GCE isotherms exhibit hysteresis loops, which are confined between the gas-like and liquid-like spinodal points. One interesting feature, not previously recognized, is that the condensation in the MCE isotherms for pores of different topologies is identical, and it occurs at the same chemical potential, which is the coexistence chemical potential of two phases in equilibrium: the liquid-like condensate and the gas-like phase. At the onset of the condensation, the liquid-like condensate is a thin liquid bridge formed by taking molecules from the metastable adsorbed film, resulting in a thinner and stable adsorbed film. As more molecules are added into the pore, the liquid bridge thickens along the axial direction and both menisci advance toward the pore mouth. As the menisci approach the pore mouth, the liquid condensate is progressively restructured to become more cohesive. When the pressure is reduced in the grand canonical ensemble (open system), a lower pressure is required to disrupt this cohesive structure, resulting in hysteresis in the closed end pores. The classical theories, which conclude that isotherms for closed end pores must be reversible, are in error because they assume that the interfacial energy parameter (the product of the surface tension and the liquid molar volume) has the same value as in a bulk liquid. (C) 2015 Elsevier B.V. All rights reserved.