Due to the significant development of new materials with controlled shapes and sizes in the last decades and their great potential in many applications in chemical and energy industries, the need for a better characterisation for pore morphology and pore size is more essential than ever. These parameters are mainly derived from the analysis of an experimental adsorption isotherm. Since condensation and evaporation are sensitive to pore size, the portion of the isotherm in the region of condensation and evaporation is analysed for the required parameters. More often than not the isotherm in this region exhibits a hysteresis loop enclosing stable and metastable states, and the shape and size of this hysteresis loop are complex functions of temperature, pore size, molecular probe and to a lesser extent the properties of the adsorbent. Although there are a number of classical theories (including DFT) and molecular simulation work, none has shown convincingly the microscopic origin for the condensation and evaporation. Therefore the aim of this research is to provide a microscopic mechanism of condensation and evaporation, and we achieve our goal with a molecular simulation work of gas phase adsorption on a number of mesoporous solids whose pores are uniform cylindrical, non-uniform or interconnected to develop a functional relationship of the hysteresis characteristics in terms of temperature, adsorbate, adsorbent structure and morphology. We have shown that position, shape and size of a hysteresis loop does not solely depend on the pore diameter but also on the pore length, the presence and position of the closed end and the type of connectivity. A significant finding from this thesis is that adsorption isotherm for a closed end pore is not always reversible but it can be hysteresis when its pore diameter and pore length are large enough. The thesis also contributes a systematic understanding of condensation/evaporation and hysteresis of gas adsorption in mesoporous materials at molecular level. The results help not only to substantially improve characterisation methods using gas adsorption and molecular simulation but also to better design adsorptive processes in separation and purification.