Spreading vinasse on soil enhances the risks of anaerobiosis and changes in soil cation exchange capacity (CEC). Our aims were to (i) compare CEC measurement methods in different aeration conditions, and (ii) affect variations in CEC to changes in soil pH, reductive dissolution of oxyhydroxides, and reduction of clay iron. In a first experiment, incubations of ferralsol slurries were performed according to the sequence aerobiosis (7 d) – anaerobiosis (28 d) – aerobiosis (28 d), rum vinasse being supplied at the beginning of anaerobiosis. CEC was measured using cobalthexamine, Cu-trien or Mg2+; simultaneously, we characterized mobilized metals, exchangeable cations and slurry pH. In a second experiment, soil CEC was measured from the elution of NH4+ at several pH (5, 5.5, 6, 6.5 and 7). In a third experiment, we assessed redox reaction between cobalthexamine3+ and Fe2+. For soil incubated aerobically during no more than 7 d, soil CEC estimates varied with the method in the order Cu-trien < Mg2+ < Cobalthexamine < NH4+. It increased of only about 2 cmol/kg, when pH increased from 5 to 7. After 28 d of anaerobiosis, CEC were 5.2, 2.4 and 1.3 times their initial estimates for cobalthexamine, Cu-trien, and Mg2+ methods, respectively. After exchange at this date, the solutions had pH similar to that of the soil slurry (6.9), except for cobalthexamine (9.4); in addition, exchanged cations agreed only with Cu-trien and Mg2+ CEC estimates, when accounting for rough estimates of exchangeable Fe2+ and Mn2+. We noted that Fe2+ can reduce cobalthexamine, leading to the release of NH3, the increase in pH, the adsorption of reduced cobalt complexes as well as the precipitation of Co(OH)2 as it was confirmed by kinetic geochemical modeling, all these facts skewing the estimate of the CEC. The return to aerobic conditions led to a quick decrease in CEC.