The high temperature mechanical behaviour of oxynitride glass-ceramics in the YSiAlON and ErSiAlON systems was studied in the 950-1150degreesC temperature range under compressive stresses ranging from 20 to 100 MPa. The parent glass had a composition of 35 Y(or Er)-45 Si-20 Al-83 O-17 N in equivalent percent. Starting from these glasses, glass-ceramics were prepared using a two stage heat treatment: nucleation at the optimum nucleation temperature followed by crystal growth at 1050, 1150 or 1250degreesC. The two parent glasses had similar viscosities, with that of the Er-glass being slightly less than that of the Y-glass. After the devitrification treatment at 1050degreesC, B-phase (M2SiAlO5N) was the only crystalline phase formed in both systems. The creep behaviour was similar for the yttrium and the erbium materials. It was characterised by a long transient stage, due to the viscoelastic response of the residual glass, with recovered strain after unloading decreasing as loading time increased. The creep resistance was compared to that of the parent glasses in terms of apparent viscosity. The crystallisation of 75% of the glass resulted in an increase in viscosity such that a temperature some 100degreesC higher showed the same viscosity value. After heat treatment at 1150degreesC, the phase assemblage in the yttrium material changed with the formation of wollastonite and partial conversion of B-phase into Iw-phase. The apparent viscosity was 2 orders of magnitude higher than that of the samples heat treated at 1050degreesC and no strain recovery was observed upon unloading. In contrast, the erbium materials retained the same microstructure as after the heat treatment at 1050degreesC and there was no difference in the creep behaviour of the samples heat treated at 1050 or 1150degreesC. After a crystallisation treatment at 1250degreesC of the yttrium parent glass, the glass-ceramic consisted of yttriun aluminium garnet, N-apatite and beta-Y2Si2O7 and showed excellent creep resistance up to 1050degreesC, but failed rapidly at higher temperatures as a consequence of fast oxidation in air.