Maleic hydrazide (3,6-dihydroxypyridazine, I) is reduced to the dicarbonyl form I at the dropping mercury electrode in two-electron waves. The first wave I-1 corresponds to a reduction of a diprotonated, the second I-2 of a monoprotonated, the third I-3 of an unprotonated and the fourth I-4 of an anionic form. Exclusion of the dihydroxy form I " and the monohydroxy form I’ as electroactive species was based on electrochemical properties and comparison of pK(a) values. Polarographic currents are limited by the rate of protonation occurring as a surface, heterogeneous process. Wave I-1 at pH 3 to 5 is the most suitable for analytical applications.