On weekends in Los Angeles County the average atmospheric concentrations of the primary reactants, NO and reactive hydrocarbons, leading to photochemical air pollution decrease. The behavior of oxidant concentration (used as a measure of the products of the photochemical reactions) usually does not follow that of the primary pollutants. In fact, we find that for most of the year the average weekend oxidant concentration is higher than the corresponding weekday value, despite the lowered emissions. However, for areas and times of particularly high oxidant concentration, for example at inland stations during the summer months, the oxidant levels do decrease on weekends. Thus care must be taken when designing short-term oxidant control strategies, as indiscriminant application of short-term traffic decreases to random days could be counterproductive. Such strategies, however, might be useful for days and places of particularly high oxidant concentration and further research to delineate the requisite conditions for the success of such strategies is in process.