Underway measurements of the CO2 partial pressure (pCO2) and the sea surface temperature were made in the northwestern Arabian Sea during late intermonsoon and southwest (SW) monsoon 1997. Additionally, dissolved inorganic carbon (DIC) and total alkalinity were analyzed from the surface and deeper waters. CO2 fluxes between atmosphere and ocean surface were estimated. Monsoon-forced changes in the near-surface carbonate system became clearly visible a few weeks after onset of the SW monsoon. Because of the upwelling of CO2-rich waters along the Omani coast, DIC and pCO2 locally increased up to 2210 ¿mol kg-1 and 715 ¿atm, respectively. The Arabian Sea thus acted as a CO2 source during SW monsoon. Highest CO2 fluxes of >150 mmol m-2 d-1 were observed in the upwelling region off Oman, while the open Arabian Sea revealed relatively lower CO2 fluxes but contributed more to the overall CO2 release because of its larger area. Total CO2 emissions from the Arabian Sea from May until August 1997 are estimated to amount 67.6 Tg C. The upwelling-induced impacts of biological activity on the carbon cycle were estimated by tracing freshly upwelled water along its transport way along the sea surface. The obtained CO2 uptake by enhanced net community production (NCP) is ~3.6 times higher than the CO2 emissions. In contrast to phosphate, upwelled nitrate apparently is consumed by NCP within a timescale of some 10 days, and the upwelling thus does not directly supply the central Arabian Sea with nitrate.