The western tropical South Pacific was sampled along a longitudinal 4000 km transect (OUTPACE cruise, 18 February, 3 April 2015) for the measurement of car-bonate parameters (total alkalinity and total inorganic carbon) between the Melanesian Archipelago (MA) and the western part of the South Pacific gyre (WGY). This paper reports this new dataset and derived properties: pH on the total scale (pH_T) and the CaCO₃ saturation state with respect to aragonite (Ω_ara). We also estimate anthro-pogenic carbon (C_ANT) distribution in the water column using the TrOCA method (Tracer combining Oxygen, inorganic Carbon and total Alkalinity). Along the OUTPACE transect a deeper penetration of C _ANTin the intermediate waters was observed in the MA, whereas highest C_ANT concentrations were detected in the subsurface waters of the 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 C_ANT increases by 1.1 to 1.2 µmol kg⁻¹ a⁻¹. In the MA intermediate waters (27 kg m ⁻³ < σ_θ < 27.2 kg m ⁻³) an increase of 0.4 µmol kg⁻¹ a⁻¹ C_ANT is detected. Our results suggest a clear progression of ocean acidification in the western tropical South Pacific with a decrease in the oceanic pH T 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.