A large amount of Ga atoms, sometimes nominally as high as 45% of the total group III constituents, was found in the AlInN films prepared by metalorganic chemical vapor deposition (MOCVD) under an In-very-rich environment, even when no Ga precursor was introduced. GaN template has been found to be the main source supplying most of the unexpected Ga atoms during epitaxial growth, of which about 2% might be contributed from other residual Ga-contained sources in the reactor. Secondary ion mass spectroscopy (SIMS) profile measurement shows that atom interdiffusion between In, Al, and Ga atoms is the main path of the high percentage of unexpected Ga atoms incorporating into the AlInN epilayers. A high growth temperature and the H₂ added in carrier gas may enhance the Ga atom incorporation into AlIn(Ga)N epilayer by promoting decomposition of GaN template and increasing atom diffusing ability.