This paper discusses effects of humidity on photocatalytic cyclohexane oxidation performed in an internally illuminated monolith reactor equipped with an immobilised layer of approximately 3 μm titania (Hombikat uv100). Using dry nitrogen containing 10–20% of oxygen, cyclohexanone is produced with high selectivity (>90%) over cyclohexanol. The photocatalytic monolith deactivates within 80 min of operation. Regeneration of activity of such deactivated monolith is possible by air treatment at 450 °C. When the applied nitrogen/oxygen gas is humidified, stable ketone production rates are obtained around 5 × 10−6 mol h−1 at an optimised relative humidity of 65%. Ketone over alcohol selectivity is lower in humidified conditions, the ratio of the cyclohexanol/cyclohexanone production rates increases from 0.4 to 1.0 as a function of increasing humidity from 30% to 90%. Rapid transients in water vapour content lead to relatively slow changes in concentration of in situ produced cyclohexanone and cyclohexanol. The observation of these changes is used to explain the effect of humidity on reactor performance.