Ozone and total oxidant in ambient air were measured simultaneously for a two month period in Philadelphia and a similar period in Denver, Colorado. Ozone was measured by the chemiluminescent technique and total oxidant by both colorimetric and coulometric techniques. The object was to compare these methods. The ozone and total oxidant instruments were calibrated by a constant ozone source (standardized by the neutral KI analytical technique) at the beginning and end of each experimental period. The chemiluminescent instrument operated satisfactorily more than 97% of the time, but the colorimetric total oxidant instruments operated satisfactorily less than 50% of the time. It is thus not possible to calculate ozone concentrations in urban air from measured total oxidant concentrations with any degree of confidence. The chemiluminescent ozone measurement is more specific and less subject to interference than the colorimetric or coulometric 'total oxidant' measurement. (Author)