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RE-Si-Mg-O-N glasses with RE = Sc, Y, La, Yb, Sm, or Lu with two different nitrogen contents were investigated to understand the effects of rare-earth elements and nitrogen on microhardness, indentation modulus, fracture toughness and viscosity. The microhardness and indentation modulus of glasses containing lanthanide dopants increase approximately linearly with the increase of the cationic field strength (CFS) of the corresponding RE element, whereas fracture toughness is almost unaffected. Nitrogen content increase shifts the dependencies to higher values. The properties of the glasses with non-lanthanide dopants, Y and Sc, deviate from linear dependence. At elevated temperatures, variations of the softening temperature T g from linearity were found in Yb-and Sc-containing glasses. Additional differential thermal analysis experiments on Yb-containing glass revealed the possibility of changes in glass structure at elevated temperatures. CFS concept seems to be insufficient to describe all the effects of rare-earth dopants on the properties of the oxynitride glasses.