We have investigated muonium (Mu) defect centers in single crystal AlN as an analog for atomic hydrogen impurities. A nitrogen-related muon level-crossing resonance is associated with a static center formed by trapping of a mobile Mu impurity at another defect. This trapped Mu is released above 800 K. Muon spin depolarization data imply that both Mu(0) and ground-state Mu(+) centers are mobile. Strong correlations between growth of the trapped Mu resonance and Mu(0) motion and transformation rates above 400 K imply that Mu(0) is the more likely precursor in that region. (C) 2003 Elsevier B.V. All rights reserved.