This paper details supplementary characteristics of the nano-scale inclusions recently discovered in β-sialon grains in a Sm α′-β′ composite material given a post-preparative heat-treatment at 1450°C. In addition to heat-treated materials, the features were also observed in the 'as-sintered' billet. While many inclusions were distributed randomly, some were organized into planar arrays. The nature of many features concur with those of the grain boundary glass in the 'as-sintered' material, thereby hinting that these inclusions are composed of entrapped grain boundary liquid. Furthermore, it is suggested that the inclusions may result from the α′ → β′ transformation, insofar as they may possess some of the species that constitute the difference in composition between the two phases. © 1997 Trans Tech Publications.