The spreading of vinasse may favour soil anoxia and affect cation exchange capacity (CEC). Our aims were to assess the reliability of CEC measurements in reducing conditions, quantify CEC variations in a ferralsol under changing aeration conditions, and understand the mechanisms involved. In a first experiment, soil slurry was incubated for a succession of 7 oxic, 28 anoxic and 28 oxic days, vinasse being supplied when anoxia began. CEC was measured at intervals by exchanging Mg2+, Cu-trien or Co(NH3)6 3+, the solutions being characterized before and after exchange. In a second experiment, soil CEC was measured after the elution of NH4 + solutions of pH 5 to 7. In a third experiment, we assessed the reduction of Co(NH3)6 3+ by Fe2+ and checked for induced changes by geochemical simulations. During the first oxic period, CEC varied according to method, in the order: Cu-trien<Mg2+<Co(NH3)6 3+<NH4 +. It increased by 2 cmolc kg?1 when pH increased from 5 to 7. After 28 days of anoxia, CEC estimates were 2.75, 1.43 and 5.1 times their initial values for the 0.05 MMg2+, Cu-trien and Co(NH3)6 3+ methods respectively; pH after exchange was about the same as initial slurry pH (6.9), except after Co(NH3)6 3+ exchange when it was 9.4. This complex was partly reduced by Fe2+ leading to NH3 release, pH increase, Fe(OH)3 and Co(OH)2 precipitations, Co(II) adsorption and overestimation of CEC. After the return to oxic conditions, the CEC did not return to its initial value. At all dates, CEC measured by 0.05 MMg2+ method agreed with the exchangeable cation measurements.