With respect to the photocatalytic TiO₂ powder dispersed in the water, the relationship between the experimental conditions influencing photocatalytic activities and the degree of decomposition were examined for the various sources of photocatalytic powder. For the relationship between pH and level of organic decomposition, a high degree of decomposition was shown in the range of pH 5 to 7, whereas the activity was significantly reduced out of the range. The cations and anions contained in the slurry also affected the decomposition depending on their concentrations and the sources of photocatalytic TiO₂ powder. Dissolved oxygen enhanced the degree of decomposition. This might be due to the effect of oxygen acting as an electron scavenger. The destruction yields of 4CP with the photocatalytic TiO₂ powders were affected by the wavelength of irradiated UV light on the TiO₂ powder. One particular source of TiO₂ powder has not always shown superior activities irrespective of kinds of target organics and experimental conditions. These showed that the degree of organic decomposition by the photocatalytic TiO₂ powders dispersed in the water varied significantly, depending on the environmental conditions the photocatalyst was surrounded.