We present a study of the impact of initial concentrations on the modeling of photochemical oxidants. A simple impact model is employed, and an impact factor is defined which describes the ratio between the initial concentration and the time-dependent model concentration. The calculations have been carried out with data from a box model and a comprehensive three-dimensional (3-D) model. Results are for three different sites in the California San Joaquin Valley. By using data from the chemical box model a very active chemistry and rather high concentrations are obtained since no transport processes are included. The impact of the initial concentrations is small already after 24 hours of model integration under such conditions. Results from the more realistic 3-D data show that the impact factor is reduced to less than 10% within the planetary boundary layer after 48 hours for nearly all chemical components studied. An exception is the sum of selected grouped species of Ox+NO2+NO3+N2O5+HNO3+PAN (sum of reservoir species for O3) which is not below 10% before approximately after 3 days at the two least polluted sites. In the free troposphere the impact factors of the initial concentrations are large even after 3 days for paraffin, ethene, and isoprene. For selected grouped species of Ox+NO2 and Ox+NO2+NO3+N2O5+HNO3+PAN, large impact factors are still found after 3 days. The large impact of the initial concentrations in the free troposphere strongly complicates any model evaluation by use of measurements. ¿ 2001 American Geophysical Union