Aircraft measurements of ozone (O-3) and its precursors, including NO, CO, H2O, and nonmethane hydrocarbons (NMHCs), were made over the western Pacific in the 20degrees-45degreesN latitude range in January and April-May 2002 during the Pacific Exploration of Asian Continental Emission (PEACE)-A and B campaigns. These measurements have provided data sets that, in combination with Transport and Chemical Evolution over the Pacific (TRACE-P) data taken in March 2001, enable studies of O-3 photochemistry from winter to late spring. A photochemical box model is used to calculate ozone formation (F(O-3)) and destruction (D(O-3)) rates constrained by the observed species concentrations. The values of F(O-3) and D(O-3) are controlled directly by NO, J((OD)-D-1) (O-3 photolysis frequency), H2O, OH, and HO2. Changes in HO2 concentration cause corresponding changes in both F(O-3) and D(O-3), leading to their coupling. Concentrations of these species, which are strongly influenced by photochemistry and transport from the Asian continent, underwent large seasonal variations. In the boundary layer (0-3 km), NO was much higher in January than in April-May, because of stronger winds, lower convective activities, and lower oxidation rates by OH in winter. The net O-3 formation rate, given by P(O-3) = F(O-3)-D(O-3), was largely positive in the boundary layer at 30degrees-45degreesN (1.5-4 ppbv d(-1)) in January, mainly because of high NO and low H2O values. Net O-3 formation continued from January to the end of March, demonstrating that the western Pacific is an important O-3 source region during this season. Net O-3 formation nearly ceased by late April/May because of the decrease in NO and the increase in H2O. In the latitude range of 20degrees-30degreesN, P(O-3) in the boundary layer was positive in January and turned negative by March. The earlier transition was mainly due to lower NO and higher H2O concentrations, combined with weaker transport and higher temperatures than those at 30degrees-45degreesN. The upper troposphere (6-12 km) has been shown to be a region of net O-3 formation throughout most of the year because of high NO and low H2O. The present study illustrates that a decrease in the net O-3 formation rate at 20degrees-45degreesN latitude from winter to late spring is explained systematically by the increases in J((OD)-D-1), H2O, OH, and HO2 (primarily due to increases in temperature and solar radiation) and the decrease in NO ( primarily due to decrease in transport from the Asian continent). Differences in the seasonal variation of O-3 photochemistry observed over the North American continent are interpreted in terms of the differences in factors controlling O-3 formation and destruction.