Highly porous poly(methyl methacrylate) (PMMA) with porosities ranging from 65 to 78% were synthesised at room temperature under UV light by the polymerisation in solution method with several cross-linker contents using ethylenglycol dimethacrylate. Bulk poly(methyl methacrylate) samples (nonporous) with the same cross-linker contents were also prepared as reference materials. The results of performing thermo-mechanical analysis, differential scanning calorimetry and water sorption isotherms showed that the mechanical performance and water sorption of highly porous PMMA depends on more on the microstructure (porosity) than on the material composition only in the glassy state. Furthermore, in this broad range of temperatures, the mechanical behaviour of highly porous PMMA, considered as a biphasic material composed of bulk PMMA and pores, can be perfectly predicted by Takayanagi's threeblock model. For high water activities, surprisingly, water sorption decreased considerably in highly porous PMMA as compared with bulk PMMA due to the formation of water clusters. (C) 2017 Elsevier Ltd. All rights reserved.