We have achieved significant plastic strain of MgO : n Al2O3 from 20° to 950 °C (0.1 to 0.5 Tm) by compression testing under superimposed hydrostatic pressures of 1.3 to 1.7 GPa. The easy slip system for nearly equimolar spinel (n ≃ 1.1) at low to intermediate temperatures is {110} . At temperatures up to 600 °C, the critical resolved shear stresses CRSS for this slip system are about half those for {111} and {100} , the other two slip systems which are activated. Strongly non-equimolar crystals (n ≃ 3.5) do not deform by {110} glide ; the CRSS for {100} is about the same. At temperatures below 500 °C, the CRSS for all of the slip systems in the nearly equimolar crystals decreases with increasing temperature. At temperatures above 500 °C (n = 1.1), the CRSS for {110} slip dramatically increases, and the CRSS for both {111} and {100} slip are insensitive to temperature. A number of potential mechanisms could account for these temperature effects, including a strengthening mechanism predicted earlier by two of the present authors