YSiAlON glasses have been produced using a cation ratio of Y: Si: Al = 3.5: 3.38: 2 with a nitrogen content of 17% and Y: Si: Al = 3.45: 3: 2 with varying nitrogen contents (N=8, 10, 12, 14, 17, 20 V.). Crystal phase formation occurred after heat treatment of the glasses under nitrogen atmosphere for 10 hours in the temperature range 1050-1150degreesC. Glass-ceramics were characterised by X-ray diffraction (XRD) and certain mechanical properties measured. Depending upon heat treatment temperature and/or nitrogen content, either a single crystalline phase glass-ceramic or a multi-phase glass-ceramic containing B and I-w was produced. Low nitrogen contents (8(e)/(o) N) favour the crystallisation of I-w-phase over B-phase, whereas B-phase is formed more readily for higher nitrogen contents (17 (e)/(o) N).