© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. High hole concentrations in AlxGa1-xN become increasingly difficult to obtain as the concentration of Al increases. It is well known in GaN and related alloys that hole concentration is directly affected by compensation and extended defects. Using electron paramagnetic resonance (EPR) spectroscopy, we studied the amount of neutral Mg in AlxGa1-xN with x = 0 to 0.28. 0.4-0.9 μm thick Mg-doped AlxGa1-xN films were grown by metal-organic chemical vapour deposition and annealed at 900 °C anneal in N2. EPR measurements indicate that the amount of neutral Mg decreased by 60% in AlxGa1-xN films for x = 0.18 and 0.28 as compared to x=0.00 and 0.08. Experiments also showed that the lower neutral Mg for higher Al compositions trend did not depend on threading dislocation densities in the range of 3-20x109 cm-2, capping the surface with 5 nm of P+ GaN, or detailed annealing conditions. Additional studies show that oxygen and carbon concentrations are insufficient to account for the decrease in neutral Mg observed in the samples. Although the study cannot isolate the cause for the decrease in neutral Mg, the results clearly demonstrate that the acceptor concentration decreases with increasing Al, providing an additional limitation to achieving high hole densities.