Complex formation between Fe3+ ions and malonic acid (H2L), in acidic aqueous solutions containing 0.5 M NaNO3 as inert ionic medium at 25°C, has been studied potentiometrically. The data were consistent with the following reactions and related stability constants: Fe3++L2- = FeL+ log β0,1,1 = 7.52 Fe3++2L2- = FeL2- log β0,1,2 = 13.29 Fe3++3L2- = FeL33- log β0,1,3 = 16.93 which agree partially with previous models. No mixed complexes were detected, i.e. hydrolytic complex species. A suggestion of possible structures for the complex species formed is briefly discussed. In separate experiments and in the absence of FeIII, protolytic reactions of malonic acid were studied under the same experimental conditions. A numerical comparison of these results with those already published was carried out by means of the Specific Interaction Theory (SIT). © 1989.