The primary acidity constant of maleic acid has been determined at 298.2 K in ordinary water by conductance measurements and in both ordinary water and deuterium oxide by precise e.m.f. measurements with glass and silver–silver chloride electrodes in a cell without liquid junctions. Both methods give pKH values which increase with the applied ion-size parameter a and which are identical (pKH= 2.038 ± 0.008) when 1010a/m = 5 is used. The isotope effect ΔpK = pKD–pKH decreases slightly with a but for all reasonable ion-size parameters it has an abnormally high value (e.g. ΔpK = 0.561 ± 0.012 when 1010a/m = 5) which may be due to internal hydrogen bonding in its anion. For 3⩽1010a/m⩽7 the pKH-values are significantly higher than that generally accepted, and the ΔpK values support the lower of the two previous ones.