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This paper analyzes the weekday–weekend differences of ozone, NO x (2) and non-methane hydrocarbons (NMHCs) in Tokyo and Osaka, Japan, from 2001 to 2004, in order to investigate the ozone weekend effect (OWE). Data were measured at 24 and 23 general air monitoring stations in Tokyo and Osaka, respectively. We focused on NO x and NMHCs to investigate the causes of OWE. Specifically, we examined weekday/weekend differences in the titration of O3 by NO and the photochemical ozone production. Weekend ozone concentrations were confirmed to be greater than those of weekdays during most periods in both Tokyo and Osaka, although precursor concentrations were higher on weekdays. Weekday ozone concentrations in Tokyo (other than during 2003) and Osaka decreased due to the reaction of O3 with NO, resulting in higher ozone but lower O x (the sum of O3 and NO2) concentrations on weekends than on weekends. In 2003 in Tokyo, however, the effect of the photochemical ozone production on weekends was larger than on weekdays, as determined by the comparison of ozone concentration increase versus NMHCs/NO x ratio and O x between weekdays and the weekend. The weekend NMHCs/NO x ratio would be in the boundary area between volatile organic compound (VOC)- and NO x -limited regime, so it is considered that the difference between 2003 and the other periods in Tokyo might be due to a change in precursor limitation. [Copyright &y& Elsevier]