Two types of TiO2 samples, ST-01 (Ishihara-Sangyo, Japan) and A11 precursor (Police, Poland), were heat-treated at 400-1000 degrees C and characterized by the phase composition, crystallite size and lattice strain. These TiO2 samples were tested for methylene blue (MB) decomposition and OH radical formation. Through heating TiO2 up to 700 degrees C for 1 h the single anatase phase was remained, which had improved crystallinity, large crystallite size and very small lattice strain. By extending the calcination time up to 2-5 h, the anatase phase partially transformed to rutile phase, much faster by A11 precursor than by ST-01. Transformation of anatase phase to rutile reduced the rate of methylene blue decomposition, although OH radical formation was the highest in the samples having around 9 mass% of rutile. However, methylene blue decomposition not only depended on OH radical formation on TiO2 particles, but also the content of even small amount of rutile in TiO2 reduces markedly the rate of methylene blue decomposition. (C) 2006 Elsevier B.V. All rights reserved.