The appearance of (two) cathodic peaks in the region of the second reduction process (E similar to - 0.55 V) during positive-going potential scans in cyclic voltammograms, at the HMDE, of nitroprusside (NP) (1 x 10(-3) mol dm(-3)) at pH 5.5 is evidence of a comproportionation process involving adsorption. This is supported by the fact that these inverted peaks are eliminated on the addition of surface active Triton X-100. The evidence from the present study indicates that the well-characterised tetracyanonitrosylferrate(II) ion, [Fe(CN)(4)NO](2-), is the sole reductand responsible for the second one-electron reduction process of nitroprusside at all pH values, and that the [Fe(CN)(5)NOH](2-) ion, the protonated form of the initial product ([Fe(CN)(5)NO](3-)) of the first one-electron reduction of nitroprusside, is not formed. On the other hand, at lower (sub-micromolar) concentrations, the reaction taking place at the potentials of the second reduction step of NP follows different pathways depending on the pH. The voltammetric and adsorptive characteristics of the ion [Fe(CN)(4)NO](2-) at pH 3.0 and 7.6 have been investigated by square-wave (SW) voltammetry and cyclic voltammetry (CV). Integration of the current under the CV peak at - - 0.55 V allows the number of electrons involved in the reduction at this potential to be determined : one at pH 7.6 and three at pH 3.0. A surface regenerative process induced by H+ and based on the disproportionation of the product of the second reduction step of NP is proposed in order to explain this. The disproportionation is favoured at high [H+]/[NP] concentration ratios, and, under these solution conditions, the final four-electron reduction product of NP, [Fe(CN)(4)NH2OH](2-), is produced at this low potential, rather than at the much more negative potentials required polarographically. Bulk electrolysis experiments confirmed the proposed mechanism.