Electron paramagnetic resonance (EPR) measurement under uv light irradiation was performed on Al-doped anatase TiO2 crystals grown by a chemical vapor transport method. In an as grown crystal, characteristic signals of EPR sextuplets are observed at 100-40 K. The intensity of the sextuplets decreases gradually with decreasing temperature. The sextuplets arise from a photogenerated hole of S=1/z interacting with an Al nuclear spin of I=5/2. The angular dependence of the sextuplets is successfully explained by a model where four symmetrically equivalent paramagnetic species giving the sextuplets are present in the crystal and the principal z-axis of a diagonal g-matrix for the paramagnetic species has a small deviation from the crystalline c-axis. The analysis of the angular dependence of the sextuplets gives anisotropic g-values of g(xx)=2.0254, g(yy)=2.0016, and g(zz)=2.0129, and hyperfine coupling constants of A(xx)=4.93, A(yy)=6.04, and A(zz)=5.98 G. An EPR signal from bulk Ti3+ is also observed at temperatures below 40 K regardless of uv irradiation. In an annealed crystal, the sextuplets are observed at temperatures below 70 K under uv irradiation. The EPR signal corresponding to bulk Ti3+ is absent in the entire temperature range. These experimental results differ greatly from those for the as-grown crystal. (c) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim