The main purpose of this study is to investigate various initial temperatures (100-230 degreesC) and pressures (760-2,280 mmHg) for fire and explosion characteristics of o-xylene (OX)/air mixtures, which are commercially used in the production of phthalic anhydride (PA), so that empirical flammability models can be established and used in calculations for flammability zones. According to the results of the experiment, while the experimental condition of OX is controlled by the same oxygen concentration, if the initial temperature is increased, the upper explosion limit (UEL), flammability zones of OX are correspondingly increased, but the maximum explosion pressure (P-max) gas or vapour explosion constant (Kg) of OX is decreased. In addition, if the initial pressure is increased, the explosion characteristic parameters are all increased. However, while OX is controlled by the same oxygen concentration, if the initial temperature is increased, the lower explosion limit (LEL) of OX is decreased. Nevertheless, if the initial pressure is increased, the LEL has various changes under various temperatures. In general, the LEL of OX has little change at higher temperature. From the experimental results, the minimum oxygen concentration (MOC) does not significantly vary while the initial temperature is increased, whereas it will be decreased by enhancing initial pressure. Therefore, the MOC of OX is determined by initial pressure instead of initial temperature. I t is crucially important that if the initial temperature is lower than the normal boiling point of OX (144 degreesC), then the UEL, P-max and K-g of OX are higher under 100 degreesC than 150 degreesC and even higher temperatures. Consequently, the liquid-vapour coexisting phase will demonstrate higher degree of hazard. Therefore, it is important that the loading of the volume concentration of OX should not fall into the flammability zone.