A grand canonical Monte Carlo simulation study of argon adsorption in deformable graphitic slit mesopores has been carried out, to determine the mechanisms of deformation in the different stages of adsorption and desorption. At pressures less than the condensation pressure, especially in the sub-monolayer coverage region, the pore walls are slightly compressed. This is due to the decrease in the potential energy of interaction between the adsorbate and the second graphene layer (the interaction with the innermost graphene layer remains the same) and this enhancement of the solid-fluid interaction compensates for the repulsive penalty incurred by compressing of the graphene layers. This mechanism holds, irrespective of pore size and temperature, because at low loadings the two pore walls behave like two independent surfaces. At higher loadings, after condensation has taken place, adsorbate molecules in the interior of the pore attract the pore walls, while those close to the surface repel them. As a result the pore can either contract or expand at high loadings, depending on the balance between these two mechanisms. Across the capillary condensation, the attraction of the condensate in the pore interior is greater than the repulsion by the adsorbate close to the surface, resulting in pore contraction and a corresponding sharp decrease in the solvation pressure. After the capillary condensation, the pore either expands or contracts, depending on the balance between these two processes, which is a function of pore width and temperature, which in turn determine the commensurate or incommensurate packing and thermal motion of the molecules. (C) 2017 Elsevier B.V. All rights reserved.