Ozone chamber testing of rubbers occupies a prominent place in the field of polymeric degradation testing. In such testing the rate of ozone cracking of rubber vulcanizates is a function of the rate of collision of ozone molecules with the rubber surface, all other factors constant. However, the conventional mode of expression of the ozone concentration in chamber testing is on a volume O3 per volume air basis, i.e. parts O3 per hundred million (108) air by volume, abbreviated by pphm. In this paper we show that at equal ozone concentrations expressed as pphm, cracking rate is a function of the ambient atmospheric pressure (in the chamber). Thus variations in ozone test results may occur in intra- or inter-laboratory testing if ambient atmospheric pressure at the time or place of test is sufficiently different. A mode of expression that avoids this is recommended, i.e. the partial pressure of O3 in mPa.