The origin of holes in Mg-doped In xGa 1-xN films is investigated using electron paramagnetic resonance (EPR) spectroscopy. Unlike in Mg-doped GaN, the number of Mg-related acceptors in films with x between 0.021 and 0.112 decreases as the hole density increases. The EPR results indicate that the previously suggested models involving isolated acceptors or band formation cannot adequately explain hole production in InGaN. Rather, additional features such as In-induced passivating centers must be introduced. © 2012 American Institute of Physics.