Commercial ozone (O3) sensors were calibrated at the Institute for Environment and Sustainability of the Joint Research Center in Ispra (Italy). The sensors were exposed to ozone at different concentrations, ranging from 0 ppb to 110 ppb, in a climatic chamber under controlled conditions. The actual ozone concentration was not calculated from the gas control system but it was measured with standard reference methods. Some sensors were operated at constant heating power and one sensor was operated by switching on and off the power every 30 seconds. This allowed the extraction of different response curve features such as the slope of the resistance curve during the power cycle, the final resistance after that power cycle, or the difference between initial and final resistance after a power cycle. The measurement chamber was used to control different ambient parameters that could affect the sensor response such as temperature and relative humidity. All these different features and ambient parameters were combined and used in the calibration. Several models with different parameters were compared and the best one was selected. In addition to the calibration several characteristic sensor parameters were measured or calculated: hysteresis, drift, interference with different gases introduced in the chamber together with ozone: NO2, NO, CO, NH3. The performance of the sensors operated at constant heating power and the sensor operated by heating power switching on/off were compared.