The topic of this work is a novel approach for the determination of mechanical properties of thin films on a substrate based on the theoretical modeling of spherical indentation into a film substrate system together with its adequate experimental realization. First, some results of a novel analytical solution were compared to results of finite element (FE) calculations and to flat punch models. Then, SiO2 layers on silicon were investigated as an example. The measured load-displacement curves for spherical indentations were compared to curves simulated by means of the theoretical model. When using appropriate elastic parameters of the film and substrate a complete agreement can be achieved. Finally, the onset of plastic deformation within SiO2 was determined by a multiple partial unloading procedure with a 4 mu m sphere. It was found that the load necessary to start plastic deformation in a 538 nm SiO2/Si system is only 60% of the value that was obtained for a 2007 nm SiO2 layer, although the plastic hardness is the same. Using the theoretical model it can be shown that the plastic deformation starts within the him and that - despite the different critical loads measured - the critical von Mises stress is the same.