A series of silicon-aluminum oxynitride-glass compositions containing selected rare-earth (RE) additions were prepared to examine the effects of specific RE, as well as Si:AI:RE and N:0 ratio, on properties. The properties that were characterized included density, thermal expansion coefficient (alpha), glasstransition temperature (T-g), hardness (H), and Young's modulus (E). The compositions (in equivalent percent) selected included: 55 Si-20 RE-25 Al oxide and 80 0-20 N oxynitride, and 45 Si-30 RE-25 Al oxide and 70 0-30 N glasses. The results show that the density increased significantly with an increase in the RE atomic mass and slightly with an increase in N:O ratio. For each of the fixed Si-Al-RE-O-N compositions, the E, H, and Tg values were each increased by substituting smaller RE ions, whereas the alpha value was decreased. For each specific cation composition and RE, increasing the N:O ratio systematically led to a decrease in the a values but an increase in the E, H, and T-g values. The observed response in the glass properties to changes in composition appears to reflect modifications in the bonding within the glass network.