We have used the 3D photochemical model CMAQ to simulate the ozone weekend effect, a phenomenon in which urban areas can have higher ozone concentrations on weekends than on weekdays even though NOx emissions are usually lower on weekends. A simulation containing a weekend is compared to hypothetical simulations in which the anthropogenic emissions for the weekend have been replaced by weekday emissions. The simulations are identical in all other respects. Process analysis is used to explain the results. We find that the weekend effect can be decomposed into an ozone titration component and an odd oxygen component, each contributing about half of the excess weekend ozone. The titration component simply requires that there be lower weekend NOx emissions. The odd oxygen component additionally requires that on weekends there be a higher rate of OH + (VOC or CO) reactions, brought about by one or more of lower nitric acid formation, higher OH formation from O3 photolysis, and higher VOC emissions. This causes higher weekend peroxy radical formation. The odd oxygen component also requires sufficiently high NO concentration even on the lower-NOx weekends to propagate this higher rate of peroxy formation back to higher weekend OH formation.