An all-solid-state contact electrode selective for sulfite/hydrogensulfite was constructed using a wire coated with a mixture of graphite, epoxy and hardener and covered by 4-methylpiperidinedithiocarbamate-mercury(II) complex. Its preparation and the response characteristics were examined. With an optimum thickness of the complex at the surface of the all-solid-state contact (which appears to affect the response time, slope and lifetime), the electrode exhibited a short response time and high selectivity for reducing species, and also was found to be reproducible for 3 weeks when kept in distilled water or dry. For electrodes exposed to solutions containing high levels (0.1 mol dm(-3)) of sulfite/hydrogensulfite or reducing species, e.g., Sn-II or Cu-I, the response was significantly lower. The observed response of the electrode only to reducing species can be attributed to a heterogeneous redox reaction (sulfite in the sample reduces Hg-II to Hg-I at the electrode-sample interface). Under controlled conditions such a heterogeneous redox reaction resulted in a highly selective and reproducible response to reducing species, especially to sulfite/hydrogensulfite. The response of the all-solid-state contact electrode was linear with a slope of -70 +/-5 mV between 0.1 and 5 x 10(-6) mol dm(-3) sulfite/hydrogensulfite concentration and the useful pH range from 0.1 to 1 x 10(-6) mol dm(-3) sulfite/hydrogensulfite was between 3.0 and 9.0. Further, the selectivity of sulfite over iodide, thiocyanate, thiosulfate and bromide was much improved compared with that for a liquid membrane sulfite electrode.