A reliable procedure for the simultaneous determination of ultra-trace levels of cobalt and nickel in sea water is proposed, in which the complexes of cobalt and nickel with dimethylglyoxime are analysed by cathodic stripping square-wave voltammetry based on adsorptive collection at a hanging mercury drop electrode. After comparing differential-pulse cathodic stripping voltammetry with cathodic stripping square-wave voltammetry for the determination of cobalt and nickel, the latter technique was adopted because of its inherently greater sensitivity and faster scan rate. The variation of the resolution between the cobalt and nickel peaks and of the peak currents with ligand concentration, buffer concentration, pH, adsorption time and potential, and of the instrumental parameters such as scan rate, pulse height and frequency was studied. The effect of surfactants and foreign metal ions was also examined. The advantages of the proposed method are rapid determination, high sensitivity and good precision. The detection limits for cobalt and nickel are approximately 8 pM and 0.05 nM, respectively. The applicability of this method to the analysis of sea water was assessed by the determination of cobalt and nickel in Mediterranean sea water samples.