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Ga1–xMnxN epilayers and p-i-n device structures have been grown by metalorganic chemical vapor depo-sition. Optical studies were found to investigate the role of Mn concentration and the role of different co-dopants to elucidate the origin of the room temperature ferromagnetism in Ga1–xMnxN epilayers. Increas-ing Mn concentration was found to significantly affect long-range lattice ordering. This observation was supported by the existence of a disorder-induced Raman mode at 300 cm–1 and local vibrational modes (LVMs) at 669 cm–1 that has been attributed to the formation of self-compensating nitrogen. The E1(TO) phonon frequency was found to linearly increase with Mn composition, which is expressed by (558 + 2.7x) cm−1. The intensity and the linewidth of an absorption band near 1.5 eV was found to increase with Mn concentration, and is assigned to the e to t2 transition in the split 5T2 band of Mn 3+. No absorption was detected in Si co-doped Ga1–xMnxN. In this case, a significantly increased EPR signal of Mn 2+ ions con-firmed the trapping of electrons provided by the shallow Si donor and reduced the available states for fer-romagnetic exchange. A change in the measured magnetization is observed under ultraviolet illumination