Quantitative analytical electron microscopy study of dopant distribution in the rnicrostructure of selected Nd-alpha-sialon samples revealed the presence of relatively large amounts of glassy phase at quadruple pockets, which exhibited a common composition similar to a melilite solution. Al segregants were depleted from adjacent grain boundaries to satisfy the "stoichiometry" of such glass. Existence of this glass results in significant deviation of the Nd-alpha-sialon composition from the expected values, which shifts the alpha-beta-sialon phase boundary. Only extra Nd2O3 additives enable a monolithic alpha-sialon microstructure. The absence of similar glass in Yb-alpha-sialon materials keeps the phase relations from such deviations.