The crystallisation of M-SiAlON glasses (M = Y, Ln) is particularly sensitive to small variations in composition and heat treatment temperature. The formation of I-w-phase in the YSiAlON system has been studied but little information concerning its nucleation, thermal stability, mechanical properties and extension into the lanthanide sialon series is available. In order to better understand the synthesis and to characterise more fully the I-w-glass-ceramic materials, a series of glass compositions have been prepared and characterised. These include M3Si3Al2O12.15N0.90 (M = Y, Er, Ce, Yb); M3.45Si3Al2O12.76N0.95 (M = Y, Er); Y4Si3Al2O13.50N and Y4Si3.37Al1.50O13.50N. Heat treatments were performed on these glasses and the resulting crystalline products have been studied by X-ray diffraction (XRD). Differential Thermal Analysis (DTA) in combination with XRD analysis of Y3.45Si3Al2O12.76N0.95 and Ce3Si2Al2O12.15N0.90 parent glass-compositions was used to investigate crystal phase formation. Young's modulus (E), hardness (H-v and fracture toughness (K-IC) of the glass-ceramics were measured. Glass-ceramics containing I-w-phase as the only detectable crystalline phase have been produced from M3Si3Al2O12.15N0.90 and M3.45Si3Al2O12.76N0.95 (M = Y and Er) compositions. No equivalent phase was found in the Ce- and Yb-sialon compositions.