Four cobalt complexes (cobalt(II) acetylacetonate and 8-hydroxyquinolinate and cobalt(III) 8-hydroxyquinolinate and diethyldithiocarbamate) were tested as mediators to modify carbon paste electrodes with a view to obtaining a catalytic effect for the hydrazine. Cobalt(II) acetylacetonate was found to exert no catalytic effect as the likely result of ligand having to stabilize higher oxidation states of Co potentially involved in the catalytic process. The effects of the electrolyte and pH were examined and found to be important. The proposed chronoamperometric determination for hydrazine features limits of detection at the 99% confidence level of 1.6 mM with cobalt(III) diethyldithiocarbamate, 1.5 mM with cobalt(III) 8-hydroxyquinolinate and 2.0 mM with Co(II) 8-hydroxyquinolinate. The RSD for 2 mM hydrazine was always less than 8%.