TiO2 films with thicknesses (d) above 15 nm were grown on optically polished surfaces of MgO (001) substrates held at 400 degrees C by sputtering a Ti target with an argon-ion beam when the partial pressure of O2 was kept at 1.1 x 10(-2) Pa. X-ray diffraction patterns show that TiO2 films with d < 56 nm are composed of an a-axis anatase-type structure, whereas those with d > 56 nm are composed of a mixture of phases with the c-axis parallel to the film surface. The thickness dependence of the infrared reflection-absorption spectra shows that TiO2 films with d < 56 nm are composed of both anatase and amorphous phases, whereas those with d > 56 nm are composed of anatase, rutile, and amorphous phases. The crystallinity in TiO2 films is also evaluated from the infrared reflection-absorption spectra by comparison of the observed and the calculated results determined from the dielectric function of anisotropic TiO2 bulk single crystal.