A novel experimental procedure was developed to measure oxygen mass transfer during the oxygenation of water in a thin film of a rotating disk photocatalytic reactor (RDPR). The increase in dissolved oxygen (DO) of initially deaerated water was monitored with time in the reactor vessel at different disk angular velocities after exposure of the reactor to the atmosphere. Oxygenation was predominantly achieved by oxygen mass transport through the thin liquid film carried by the disk and to a much lesser extent by direct oxygenation of the water in the reactor vessel via a surface renewal mechanism. A mathematical model was developed to simulate the phenomenon considering both cases of presence and absence of oxygen mass transport limitations. In the latter case, the model considered that the amount of liquid carried by the disk was saturated with oxygen when returning to the reactor vessel. On the basis of the model and the experimental data, it was proven that mass-transfer limitations existed until the water in the reactor vessel became saturated with oxygen. Results obtained from the model were validated by an alternative analysis using dimensionless groups characteristic to the system. The study revealed that the mass-transfer coefficient increased linearly with disk angular velocity and thus disk Reynolds number. The results showed that oxygen mass-transfer limitations decreased with increasing disk angular velocity, mainly due to an increase in the overall mass-transfer coefficient. In the presence of UV radiation, the influence of oxygen on the photocatalytic oxidation of 4-chlorobenzoic acid was investigated in the RDPR operated in batch and continuous mode. The photocatalytic reactions occurred in a thin film of liquid carried by the disk in the presence of UV radiation and ST-B01 composite spherical ceramic (SiO2/Al2O3) balls coated with anatase TiO2 catalyst. It was found that the initial degradation rate followed Langmuir kinetics with respect to oxygen concentration in the gas phase. When the oxygen concentration in the gas phase surpassed that in air, the degradation rates did not improve significantly, suggesting that operation with air instead of oxygen is most probably a more realistic practical choice. Measurements of DO during the presence and absence of UV radiation suggested that the photocatalytic reactions were mainly oxygen concentration-limited rather than oxygen mass-transfer-limited.