As part of a cooperative effort of the Joint Global Ocean Flux Study (JGOFS) and of the World Ocean Circulation Experiment (WOCE) program, we have measured total CO2 (TCO2) and total alkalinity (TA) along three sections in the northern Indian Ocean. One section through the Gulf of Aden to the Arabian Sea is parallel to the coast of Yemen. One section is across the Arabian Sea along the nominal 9N latitude and the other section is across the Bay of Bengal along the nominal 10N latitude. The measurements were performed on board RN Knorr in September-October 1995. The primary purpose of this work is to understand the penetration of anthropogenic CO2 along these ocean sections. Here, we present a novel approach to the calculation of anthropogenic CO2 in the ocean based upon the fundamentals of water-sources mixing. Consequently, we first describe the observations and mixing of water-sources before we describe the quantification of anthropogenic CO2 concentrations in these waters. The data show large spatial variations in surface seawater of both total CO2 (up to 50 mu mol kg(-1)) and total alkalinity (up to 40 mu mol kg(-1)). The variations are mainly associated with physical processes characterized by water masses of different temperature and salinity. For example, at depths we observed low TCO2 concentration at longitude 54E +/- 2E associated with the low-salinity water mass flowing northward. The contrasts between the sections across the Arabian Sea and the Bay of Bengal emphasize the large property differences between the two ocean basins. Multiparametric analyses on the data clearly show the relative contributions of different water-sources in each of the ocean sections. The mixing coefficients calculated from the multiparametric analyses are further used to quantify anthropogenic CO2 concentrations in each water-source. The results indicate that the surface water-sources contain 47.8, 42.1 and 50.4 mu mol kg(-1) in the Gulf of Aden, the Arabian Sea and the Bay of Bengal, respectively. In the surface waters there is slightly more anthropogenic CO2 across the Bay of Bengal than across the Arabian Sea. In contrast, anthropogenic CO2 has penetrated significantly deeper in the Gulf of Aden than in the Arabian Sea and in the Bay of Bengal.