Tailoring glass compositions can raise the viscosity of SiREAI oxynitride glasses. In the present study, viscosity data are obtained by determining the compressive creep response of bulk glasses in air. The findings reveal that increasing both the nitrogen-to-oxygen and the yttrium-to-aluminum ratios of the glasses shifts the glass viscosity to higher temperatures. In addition, the substitution of progressively smaller rare earths in the glass composition results in a further increase in the glass viscosity. These effects have important implications in the creep resistance of silicon nitride ceramics where the amorphous intergranular films are a major factor in creep resistance.