Ambient ozone (O 3) concentrations tend to be higher on weekends than on weekdays in urban areas, a phenomenon known as the weekend effect. Nevertheless, its downwind influence could be diverse. The MM5-CMAQ model has been used to assess the causes of weekday/weekend O 3 differences in the north-eastern Iberian Peninsula during an episode of photochemical pollution covering the whole Western Mediterranean Basin (13-16 August 2000). Emissions were estimated with EMICAT2000 model. The key differences between weekday and weekend on-road vehicle emissions are in magnitude and timing. On-road traffic accounts for ∼57% of the anthropogenic NO x and VOC emissions in the modeling domain, and are principally located along the coast and mainly in the Barcelona Geographical Area. The drop of heavy-duty tracks traffic on weekends involves a 22% reduction of NO x emissions and just a slight change in VOC emissions. Changes in the timing of emissions are directly related to differences in weekday/weekend traffic profiles, being emissions shifted 1-2 h later on weekends. The response of both ambient and simulated O 3 concentrations to day-of-week differences in emissions varies by location. The combination of VOC-sensitive regimes and NO x-titration in urban areas as Barcelona, in addition to the different magnitude and timing of emissions (decreasing NO x/VOC ratios on weekend mornings) causes the raise of O 3 on weekends (+54%). In non-urban regimes, principally associated to NO x-sensitive chemistry, a lower concentration of O 3 is observed in non-labor days (decreases of -10% in downwind areas). Rural locations, dominated by medium-long range transport, depict similar O 3 concentrations. Both discrete and categorical model evaluations are shown in order to test the accuracy of the model for representing weekdays/weekends differences within the air basin. This work helps identifying the major causes of the weekend effect in the considered domain, as the changing in mass and time of precursors emissions, and may be a useful tool to reduce ambient O 3 levels.