The photochemical treatment of water using 185 nm radiation forms the basis of an advanced oxidation process (AOP) that does not require chemical addition. The 185 nm photolysis of water generates the hydroxyl radical (OH) able to degrade trace organic contaminants. However, the strong absorbance of water (aH2O) at 185 nm imposes severe geometrical constraints on reactor design. Investigation of the reported temperature dependence of aH2O on treatment efficiency was conducted between 5 and 35 °C, in model solutions using carbamazepine as a radical probe. Comparison was made with the temperature dependence of the 254 nm photolysis of hydrogen peroxide under similar conditions. It was found that the 185 nm AOP is less sensitive to temperature under the conditions tested, suggesting the absorbance of water has a negligible effect in this range. This is postulated to be due to the fundamental structure of liquid water, whereby 185 nm photons are absorbed by a small population of interstitial H2O monomers, with no hydrogen bonds to impede the escape of OH from the solvent cage.