AlN possesses a moderate strength, high thermal conductivity and a low thermal expansion that facilitate its use in demanding thermal applications such as heat spreaders in power modules and substrates for high power density heaters, where the mechanical and thermal properties are critical. During these applications, AlN could be exposed to environments that could cause it to undergo oxidation or hydrolysis. While the oxidation and hydrolysis mechanisms of AlN have been well described in the literature, their impact on the flexural strength and thermal conductivity are less well documented and often contradictory. AlN bar, disc and square coupon samples (1.67 mm thickness) were manufactured from a tape cast green AlN with Y2O3 and Al2O3 sintering aids. The as-manufactured AlN had a characteristic flexural strength of 259 MPa and a thermal conductivity of 191 W m-1 K-1. To evaluate the effect of oxidation, samples were oxidised at temperatures between 800 °C and 1200 °C, for up to 12 hours. With oxidation below 1000 °C, no oxide layer was observed on the surface of the samples and there was a large variation in characteristic flexural strengths. Above 1000 °C, a layer of Al2O3 was produced on the surface of the AlN samples, which reached a thickness of 10 μm after 12 hours at 1200 °C. At 1200 °C, a decrease in strength was observed, down to 216 MPa after 12 hours. Following oxidation in static air, this strength decrease was greater, dropping to 182 MPa after oxidation at the same time and temperature. The presence of air flow during oxidation was reported to have a significant impact on the oxidation process, with oxide layers produced on the surface in static air an order of magnitude thicker than those produced in flowing air. No significant change in the thermal conductivity of oxidised samples was observed. The mechanism of the hydrolysis of a bulk AlN was investigated with immersion tests carried out at room temperature, 45 °C, 65 °C and 85 °C, respectively. At all temperatures, the reaction products are proposed to be Al(OH)3 and amorphous AlOOH. A significant temperature dependence on the morphology of the reaction products was observed, yet unlike for the hydrolysis of AlN powders, no change in major reaction product was observed above 80 °C. Following immersion at room temperature, a slight increase in flexural strength was reported. In contrast, immersion at all elevated temperatures led to a drop in flexural strength due to the formation of defects in the product layer. The thermal conductivity decreased with immersion at all temperatures with the morphology of the product layer having a more significant role than the intrinsic thermal conductivity of the phases present.