The sorption of fluids in mesoporous silica is an important physical phenomenon with a wide range of applications. Traditionally, mesoporous materials have been considered as inert scaffolds for the sorption and condensation reaction of the fluid. Here we present in situ small angle X-ray diffraction experiments providing evidence for a sorption strain induced in the solid that manifests itself as a change in the lattice parameter of the ordered mesopore array as the pores gradually adsorb fluid material. The experimental data are analyzed by means of Monte Carlo simulations carried out in a grand canonical ensemble describing a fluid confined by deformable substrates. We show that-in agreement with experimental data-sorption of a nonpolar fluid causes the pores to expand initially, to shrink abruptly when capillary condensation sets in, and to expand again as more liquid-like fluid is adsorbed subsequently. We show that the pore pressure can be extracted from a thermodynamic analysis of sorption isotherms in the liquid-like regime and that this information can be used for an estimation of the Young's modulus of the porous silica material. In addition, our Monte Carlo simulations indicate that the phase behavior of confined fluids is considerably changed by the deformability of the confining solid. This is reflected by a change of the location of phase boundaries at sufficiently subcritical temperatures.