The hardness of multiple glasses in the Y-Si-Al-O-N system has been determined as a function of the indentation load. The measured hardness significantly depends on the load, which implies a substantial influence of the indentation size effect (ISE). The measured hardness has been deconvoluted in a load-dependent part and a load-independent hardness using the method of Bull et al. The influence of the overall composition on the hardness of these glasses has been investigated. It is shown that, by increasing the nitrogen content, a load-independent hardness of over 9 GPa can be achieved, which is far higher than most common glass systems. (C) 2004 Kluwer Academic Publishers.