Significant seasonal variations in the concentrations of several trace gases were observed in the southern tropical Pacific during the Pacific Exploratory Mission (PEM) TROPICS-A (PT-A, September/October 1996) and PEM TROPICS-B (PT-B, March/April 1999) campaigns. The objective of this study is to evaluate the relationship between the measured and modeled seasonal variabilities through comparisons of these observations with model calculations. A three-dimensional (3-D) global-scale chemical transport model was employed to (1) evaluate the seasonal variations in ozone and CO mixing ratios; (2) calculate an ozone budget and identify its controlling factors; (3) evaluate the effectiveness of atmospheric transport barriers in the model, such as the Intertropical Convergence Zone (ITCZ); and (4) test the sensitivity of ozone over the tropical south Pacific to biomass-burning emissions. The model reproduces the main features of the observed seasonal variation in ozone and CO. According to the model calculation, the ozone burden during the PT-A was larger due to the transport of ozone into the central pacific middle troposphere from the west, whereas a net outflow of ozone from this region to the east resulted in a reduction of ozone burden during the PT-B period. Transport of ozone from the Northern Hemisphere into this region was found to be much larger during the PT-B period compared with the PT-A period. The in situ production and loss of ozone calculated in the model agreed with that derived from measurements. The gradients in the model for selected trace gases across the ITCZ were shown to be qualitatively similar to measurements.