The present work explores the glass-forming region in the Ca-Si-O-N system, particularly in the N-rich part of the system. Bulk oxynitride glasses in the Ca-Si-O-N system were prepared by melting mixtures of CaH 2 , SiO 2 and Si 3 N 4 powders in a nitrogen atmosphere at 1500-1650 • C. The glasses were characterized by X-ray powder diffraction, differential thermal analysis, scanning electron microscopy and transmission electron microscopy. Glass compositions were calculated from analyses obtained by energy dispersive X-ray (EDX) spectroscopy and combustion analysis, for cation and anion compositions, respectively. The glasses were found to be X-ray amorphous and gray to black in color. The glasses retain up to 58 e/o of nitrogen and 42 e/o of calcium. The glass formation depends on reaction kinetics and the precursor used. A strong exothermic reaction is observed at 850-1000 • C, leading to formation of amorphous and crystalline oxynitride phases that melt at high temperatures upon further heating. Glass transition temperatures (T g ) were observed to vary between 798 • C and 1050 • C, and crystallization occurs typically 130 • C above the glass transition temperature. The glass densities vary between 2.79 g/cm 3 and 3.25 g/cm 3 .