The microhardness of as-deposited and illuminated thin films of GexSb40-xS60 glasses has been measured. The volume of the micro-voids, their formation energy and the module of elasticity are determined using the free-volume theory and are considered in terms of average coordination number Z. The compositional dependences of these quantities show a well expressed peculiarity around Z = 2.67, indicating the existence of a topological phase transition. An increase of the microhardness and the elasticity in the illuminated films is observed as a result of the photoinduced structural changes.