Abstract A new technique has been developed for the accurate and precise determination of the stable Cd isotope composition of seawater. The method utilizes a 110Cd– 111Cd double spike, and it involves separation of Cd from seawater by column chromatography and isotopic analyses by multiple collector inductively coupled plasma mass spectrometry. As a by-product, it also generates precise Cd concentration data. Repeated analyses of three pure Cd reference materials and three seawater samples yielded reproducibilities of about ±1.0 to ±1.6 ε 114/110Cd (2 SD), based on measurements that each consumed about ∼8 ng of natural Cd ( ε 114/110Cd is the deviation of the 114Cd/ 110Cd isotope ratio of a sample from the standard in parts per 10,000). This demonstrates that the new double spike technique is superior to published methods of Cd isotope analyses, with regard to the acquisition of precise data for samples of limited size. Additional experiments showed that as little as 1–5 ng of seawater Cd could be analyzed with a precision of about ±2 to ±6 ε 114/110Cd (2 SD). The accuracy of the seawater isotope data was ascertained by experiments in which a Cd-free seawater matrix was doped with small quantities of isotopically well-characterized Cd. Repeated mass scans that were carried out on purified Cd fractions of several samples furthermore demonstrated the absence of significant spectral interferences. The isotope data that were acquired for the three seawater samples reveal, for the first time, small but resolvable Cd isotope fractionations in the marine environment. Cadmium-rich intermediate water from the North Pacific was found to have an isotope composition of ε 114/110Cd = 3.2 ± 1.0. In contrast, Cd-depleted seawater from the upper water column of the Atlantic and Arctic Oceans displayed isotope compositions of ε 114/110Cd = 6.4 ± 1.1 and 6.6 ± 1.6, respectively. These observations are in accord with the interpretation that the isotope effects are due to the biological fractionation that occurs during the uptake of dissolved seawater Cd by phytoplankton.