Glass-ceramics mere produced from Y- and Er-SiAlON compositions containing the I-w phase as the only detectable crystalline phase. TEM analysis showed that these materials had fine-scale glass ceramic microstructures with crystallite sizes <1 mum and glass contents between 20% and 40%, depending on the starting powder composition: Increased yttrium or erbium content of the starting material resulted in a reduced glass content. The I-w-phase crystals in all specimens were found by EDX analysis in TEM to display a solid-solution range with a clear anticorrelation between the yttrium or erbium content and the silicon content: A possible explanation for this effect was suggested. The average crystal composition approximated a cation ratio of Y:Si:Al = 3:2:1. The composition of the residual glass was also similar in all specimens. Some small silicon-rich amorphous features were found in the microstructures, and it was believed that these had occurred because of phase separation in the glass during the crystallization process.