The composition of the organic matter oxidized within the thermocline of the Atlantic and Indian oceans has been estimated from the chemical data along the σθ 27.0 and 27.2 horizons. These estimates are based on the differences between the preformed and observed concentrations of PO4, NO3, total inorganic CO2, alkalinity, and O2. Since these isopycnal horizons are ventilated both from the north and the south, the preformed concentration estimates take into account the relative contributions of these two end-members. The proportions of these end-members are estimated on the basis of the potential temperature of the water, assuming that no cross-isopycnal mixing occurred. Except for the northern Indian Ocean, the composition of the waters entering the isopycnal horizons is established through linear extrapolation of the property-oxygen trends to oxygen saturation. The σθ 27.0 and 27.2 waters do not outcrop in the northern Indian Ocean, and these horizons receive waters spilling into the Indian Ocean from the Red Sea. Therefore the composition of the Red Sea overflow water with these densities is taken to be the northern Indian end-member. The results of this study indicate that the accepted Redfield P:N:C: −O2 ratio of 1:16:106:138 requires revision. Our analysis yields a ratio of 1:16:103:172 if the carbon value is represented by the observed increase in the total CO2 concentration. On the other hand, if the carbon value is assumed to be represented by the oxygen utilization minus the oxygen used for oxidation of NH3 with two moles of O2 per nitrogen atom, a ratio of 1:16:140:172 is obtained. Thus the P: −O2 ratio lies between 1:103 and 1:140. This discrepancy may be accounted for by the increased CO2 concentrations in the source waters as a result of the uptake of anthropogenic CO2 or by an excess demand of oxygen for oxidation of hydrogenated organic molecules. Therefore, without a firm knowledge of either the corrections for the anthropogenic CO2 effect or the hydrogen content of the biological residues, the true P:C ratio cannot be obtained. The ratio of P:CaCO3 dissolution has been estimated to be about 1:12. This indicates that the CO2 produced by the oxidation of organic carbon to that derived from the dissolution of CaCO3 is bout 10:1 at these density horizons.