Photocatalytic oxidation of cyclohexane forming the organic phase of reverse micellar systems on in situ prepared TiO2 ultrafine particles has been investigated. The cyclohexane was photo-oxidized on the TiO2 ultrafine particles to form cyclohexanone and cyclohexanol under air-saturated conditions. In the absence of O-2, H-2 was generated via reduction of water in the micellar waterpools. CO was also generated in the absence of O-2 probably via a photocatalytic side reaction of I-butanol which was added as a solvent of the precursor of TiO2 (titanium tetrabutoxide : TTB) to prepare the particles, 1-butoxyl groups of TTB or l-butanol formed via hydrolysis of TTB. The molar ratio of water to TTB, W-t (=[H2O]/[TTB]) was a major factor in controlling the photocatalytic reaction behavior of the system. When the irradiation was carried out under air-saturated conditions for 18 h, the consumption rate of O-2 increased monotonously as the value of the parameter W-t decreased, while the quantities of cyclohexanone and cyclohexanol formed decreased at W-t<10. This can be attributed to a side reaction of 1-butoxyl groups of TTB molecules which is oxidized more easily than cyclohexane. This reaction becomes prominent in the extremely low W-t range owing to incomplete hydrolysis of TTB molecules.