The western tropical South Pacific was sampled along a longitudinal 4000 km transect (OUTPACE cruise, 18 Feb., 3 Apr. 2015) for measurement of carbonates parameters (total alkalinity and total inorganic carbon) between the Melanesian Archipelago (MA) and the western part of the South Pacific gyre (WGY). This manuscript reports this new dataset and derived properties: pH on the total scale (pHT) and the CaCO3 saturation state with respect to calcite (Ωcal) and aragonite (Ωara). We also estimate anthropogenic carbon (CANT) distribution in the water column using the TrOCA method (Tracer combining Oxygen, inorganic Carbon and total Alkalinity). Along the OUTPACE transect, CANT inventories of 37–43 mol m−2 were estimated with higher CANT inventories in MA waters (due to a deeper penetration of CANT in the intermediate waters) than in the WGY waters although highest CANT concentrations were detected in the sub-surface waters of WGY. By combining our OUTPACE dataset with data available in GLODAPv2 (1974–2009), temporal changes in oceanic inorganic carbon were evaluated. An increase of 1.3 to 1.6 µmol kg−1 a−1 for total inorganic carbon in the upper thermocline waters is estimated whereas CANT increases of 1.1 to 1.2 µmol kg−1 a−1. In the MA intermediate waters (27 kg m−3 < σθ < 27.2 kg m−3) an increase of 0.4 µmol kg−1 a−1 of CANT is detected. Our results suggest a clear progression of ocean acidification in the western tropical South Pacific with a decrease of the oceanic pH of up to −0.0027 a−1 and a shoaling of the saturation depth for aragonite of up to 200 m since the pre-industrial period.