We report the effect of new sustainable inorganic phosphate glass (P-glass) flame retardants for polyamide 6,6 (PA6,6). Three P-glasses differing in chemical composition and glass transition temperature (T-g) were prepared and their flame retardant effect on PA6,6 was studied by cone calorimetry, thermogravimetric analysis, and SEM-EDX. The effect of high and intermediate Tg P-glasses on the thermal stability of PA6,6 was negligible as compared to that of the low Tg P- glass due to the hygroscopic nature of the latter. However, the char formation was independent of the P- glass composition and was observed to increase by 30% in the presence of P-glass. The low Tg P-glass composition (i.e., ILT-1) was found to be a promising flame retardant for PA6,6 at a concentration of up to 15% by weight. Cone calorimetry data showed that the ILT-1 decreased both the peak heat release rate and the total heat amount released from the PA6,6/ILT-1 hybrids, resulting in an efficient formation of a glassy char layer. In contrast to the intermediate and high Tg P-glasses of this study, SEM-EDX indicated that the ILT-1 P-glass was well dispersed in the PA6,6 matrix to yield a typical droplet-in-matrix phase morphology in the melt-blended binary immiscible P-glass/PA6,6 hybrids. (C) 2014 Society of Plastics Engineers