LaCuOSe is a wide band gap (similar to 2.8 eV) semiconductor with unique optoelectronic properties, including room-temperature stable excitons, high hole mobility similar to 8 cm(2)/(Vs), and the capability of high-density hole doping (up to 1.7 x 10(21) cm(-3) using Mg). Moreover, its carrier transport and doping behaviors exhibit nonconventional results, e.g., the hole concentration increases with decreasing temperature and the high hole doping does not correlate with other properties such as optical absorption. Herein, secondary ion mass spectroscopy and photoemission spectroscopy reveal that aliovalent ion substitution of Mg at the La site is not the main source of hole doping and the Fermi level does not shift even in heavily doped LaCuOSe:Mg. As the hole concentration increases, the subgap optical absorption becomes more intense, but the increase in intensity does not correlate quantitatively. Transmission electron microscopy indicates that planar defects composed of Cu and Se deficiencies are easily created in LaCuOSe. These observations can be explained via the existence of a degenerate low-mobility layer and formation of complex Cu and Se vacancy defects with the assistance of generalized gradient approximation band calculations.