The densities of 124 samples of seawater from stations at 35¿N in the Pacific Ocean have been measured with a vibrating flow densimeter at 25¿C. The measured densities were compared with those calculated from the equation of state of Millero et al. (1976b), derived for seawaters of constant relative compositon. Values for the excess density Δd (excess) = d(meas) - d(calc) were found to be ¿3.8¿3.0¿106g cm3 from 0 to 490m, 12.5¿4.2¿106g cm3 from 490 to 1000 m, and 17.6¿2.6¿106g cm3 from 1000 to 5834 m. The excess densities for the deep waters are in good agreement with our earlier measurements: 16.1¿3.6¿106g cm3 (Millero et al., 1976c). The values of Δd (excess) predicted by correcting for the increase of alkalinity (ΔAT), total carbon dioxide (JCO2), and dissolved silica (ΔSiO2) and nitrate (ΔNO3) in the deep waters (Brewer and Bradshaw, 1975: Millero et al., 1976c) agree with the measured values on the average to ¿5.2¿106g cm3. In the deep waters the measured values of Δd (excess) are ?6¿106g cm3 higher than the predicted values. The values of Δd (excess) were also calculated by assuming that the changes in salinity due to the added solids affect the density by the same amount as changes in weight-diluted standard seawater: 106Δd=757 ΔS(%). The values of Δ (excess) calculated from 106Δd (excess) = 37.9ΔAr+72.8ΔSiO2+47.7NO3 agree with the measured values on the average to ¿4.3¿106g cm3, independent of the depth. These results indicate that the density changes due to small changes in the composition of deep ocean waters can be accounted for by changes in the salinity due to the mass of added dissolved solids (S(%) =JM1 Δn1, where M1 is the equivalent or molecular weight and Δn1 is the change in the equivalents or moles of solute i in l kg of seawater.