Dense nearly single-phase beta'-SiAION materials (with substitutional level z approximate to 1) have been prepared by hot isostatic pressing and their high-temperature deformation behavior has been investigated using low-frequency damping and torsional creep experiments. Addition of a small fraction of AlN (approximate to0.5 wt.%) to the starting (nominally z = 1) SiAlON pow der enabled us to "balance" the excess SiO2 which likely arises from surface contamination of the starting SiAlON ponder upon exposure to atmosphere. As a result, a fine-grained beta'-SiAlON polycrystal free of residual (glassy) X-phase segregated to grain boundaries could be prepared. This microstructure is in contrast with that found for an "unbalanced" composition prepared from the same raw beta'-SiAlON powder but without the corrective AIN addition. In this latter case, residual glass (X-phase), consisting of Al-rich SiO2, was entrapped at multiple grain junctions. The presence of such a low-melting intergranular glass dominates the high-temperature deformation behavior of the dilute SiAlON material, involving marked degradation of creep resistance and significant damping relaxation due to grain-boundary sliding. "Balancing" the SiAlON microstructure with a small addition of AIN enabled us to suppress anelastic relaxation by grain-boundary sliding and to increase the creep resistance of the material by more than 1 order of magnitude.