The GasEx-2001 study took place aboard the NOAA Ship Ronald H. Brown in the eastern equatorial Pacific in February and March 2001. As part of this experiment, water column measurements were collected at noon each day near a drifting array of near-surface instruments to examine the temporal evolution of the water column chemistry. These measurements were used to construct carbon mass balance estimates during this Lagrangian type study. Over a 13-day period, the net drop in mixed layer dissolved inorganic carbon (DIC) was 6.5 ¿mol kg-1. The net precipitation during this period resulted in a DIC decrease of 1.2 ¿mol kg-1. Entrainment added 0.3 ¿mol kg-1 of DIC to the mixed layer from below giving a combined net physical effect that accounted for ~14% of the total change. Biological new production removed 1.1 ¿mol kg-1 (17%) of DIC from the mixed layer. Air-sea gas exchange had the largest impact on the DIC budget, accounting for 69% (4.5 ¿mol kg-1) of the total DIC removal from the mixed layer during this period. The estimated mean gas transfer velocity based on the DIC mass balance was 13.8 ¿ 3.6 cm hr-1 (K660 = 11.8 cm hr-1). The mean wind speed during this period was 6.0 ¿ 1.3 m s-1. This gas transfer velocity is in excellent agreement with estimates generated from atmospheric micro-meteorological CO2 flux measurements collected on the same cruise. The agreement between the oceanic and atmospheric approaches supports the validity of the gas transfer velocities determined for the GasEx-2001 experiment.