The critical and transition conditions of a gaseous explosion at constant pressure and varying pressure is investigated. The study covered the solutions of the problem in pressure-time plane, temperature-time plane, as well as pressure-temperature plane. The explosion characteristics of combustible gases using the definition of criticality as an inflection point in critical trajectory. as well as other definitions of criticality are presented. The analytical study showed that the critical parameters for constant pressure explosion are lower than those for constant volumes where theta*(tr) is lowered from 0.5 to 0.29 and theta(atr) is also lowered from 0.25 to 0.17. The sensitivity of the critical pressure to the ambient temperature and the boundary between ignition and non-ignition regions are also offered. It was also found that the critical parameters in the temperature-pressure plane were different from those obtained in the temperature-time plane or the pressure-time plane. (C) 2003 Elsevier Science Ltd. All rights reserved.