The sorption behavior of water vapor and CO2 gas in photocrosslinked poly(vinyl cinnamate) (PVCA) film was examined at 30 degreesC under atmospheric pressure. Both the water sorption isotherm and the CO2 sorption isotherm obtained with quartz crystal microbalance (QCM) method obeyed the simple Langmuir's equation. Water vapor/CO2 mixed-gas sorption isotherms were also obtained. Total amount of sorbed mixed gases was clearly influenced by the partial pressure of water vapor (p(w)) and CO2 gas (p(c)) in the atmosphere. A modified Langmuir's equation based on a dual-site model was employed for predicting the competitive adsorption isotherm, and the isotherm was clearly described by the equation. The theoretically estimated amount of adsorbed water at the constant p(w) decreased slightly with increasing p(c). The effect of this phenomenon on the sensitivity of the capacitive-type relative humidity sensor was examined. As expected, the electrical capacitance of the sensor at the constant relative humidity decreased because of the coexistence of CO2 gas. However, the influence was quite small in the CO2 concentration range in the ordinary environment.