Hydrazine compounds can be determined by voltammetry, based on their hydrogen catalytic wave in formaldehyde-platinum-sulphuric acid media. Adsorption of the platinum-formazone complex on the hanging mercury drop electrode enhances the sensitivity, allowing convenient measurement of micromolar and submillimolar concentrations of simple hydrazines. The selectivity is better than that of conventional (anodic) measurements of hydrazines, as oxidizable compounds do not interfere. The sensitivity is also greatly improved. The hydrazine response is evaluated with respect to preconcentration time, concentration dependence, platinum and formaldehyde concentrations, reproducibility and other variables. The relative standard deviation (at 2 x 10(-5)M) is 1.6%.