The major factors affecting the diurnal variation of gaseous hydrogen peroxide were studied using a one-dimensional vertical gas-phase chemistry/transport screening model. The model which included diurnal variations of the inversion layer, surface emissions, dry deposition of air pollutants, and meteorological conditions such as solar radiation, temperature, relative humidity and wind speed, was used to evaluate the influence of the ratio of NMHC/NO x , the emission rates of NO x and NMHC, the deposition velocity of H 2O 2, and the height of inversion layer on the ground level gaseous H 2O 2 concentrations. The model was found to yield reasonable agreement with field data from the Carbonaceous Species Methods Comparison Study at Glendora, California, in 11–21 August 1986. Specifically, model predictions and field results all indicated that during clear skies, ambient H 2O 2 concentration was highest at about early afternoon when O 3 concentration was highest and NO x was lowest. It was concluded that the predicted gaseous H 2O 2 concentration is most sensitive to the emission rate of NO x and the ratio of NHMC/NO x , but it is less sensitive to the deposition velocity, height of inversion layer, and the emission rate of NMHC.