Oxygen is a common substitutional impurity in both AlN and GaN crystals. In the wurtzite 2H phase it can be present in AlN up to concentrations of I x 10(21)/cm(3) while in GaN it can reach concentrations of 3 x 10(22)/cm(3). These high concentrations of oxygen affect the luminescence, the optical absorption, the thermal conductivity, and the crystal perfection. The effects are somewhat similar in AlN and GaN. Representative experimental data will be presented to demonstrate the similarities, and to show how the oxygen content may be estimated from these property measurements. (C) 2002 Published by Elsevier Science B.V.