Polycrystalline LaTaON2 samples were prepared by the ammonolysis of LaTaO4, where the oxide preparation temperatures (700, 900, and 1,400 A degrees C) and ammonolysis temperatures (850, 950, and 1,000 A degrees C) were varied to control the specific surface area and porosity. The resulting LaTaON2 samples had a surface area of 15-20 m(2)/g and a pore volume of 0.045-0.059 cm(3)/g, as determined from the nitrogen adsorption-desorption isotherms. In particular, the ammonolysis of crystalline LaTaO4 (prepared at 1,400 A degrees C) can be described as a condensation from layered perovskite to three-dimensional perovskite, where mesoporous LaTaON2 were produced with pore diameters of 5-50 nm and particle sizes of a parts per thousand 1 mu m. Diffuse-reflectance measurements demonstrated that these ammonolysis parameters also affected the optical band gap energy of LaTaON2, which varied from 1.93 to 2.07 eV. Increasing the LaTaO4 preparation temperature and/or ammonolysis temperature tended to decrease the band gap of LaTaON2.