The influence of relative humidity on NO oxidation over commercial TiO2 (47 m(2)/g catalyst) was investigated. The experimental methodology followed the protocols described in ISO-22197: 2007 (E) standard. The adsorption characteristics of oxygen and H2O were monitored by adsorption calorimetry. Oxygen adsorption was low; on the contrary water vapor exhibited the characteristics of multilayer adsorption. In the first monolayer, the heats of adsorption of water sharply decreased from 180 to 80 kj/mol till about 10 mu mol/g catalyst surface coverage. Constant adsorption heats of about 80 kJ/mol were measured for coverage of about 350 mu mol/g catalyst, corresponding to a monolayer. Adsorption further continued with a constant heat of about 40 kJ/mol corresponding to the condensation enthalpy of water at the measurement temperature of 50 degrees C. The photocatalytic NO oxidation rate expression was determined as No-r alpha cN(NO)(-00.12) c(H2O)(0.38). (C) 2015 Elsevier B.V. All rights reserved.