The reactivity of highly sulphidic tailings (approximate to 53% pyrite) used in a cemented paste backfill (CPB) was investigated at the Laronde mine (Quebec, Canada). Oxygen consumption (OC) tests were performed directly in a mine slope filled with CPB. In situ OC test results showed a high oxidation rate at the beginning of the testing period (mean value of 2.4 mol O-2/m(2)/day). However, the observed oxidation rate decreased progressively to reach a flux value of 0.2 mol O-2/m(2)/day after 80 days. The physical and mineralogical characterization of the CPB and the pore water quality evolution indicated that this reduction in the oxidation rate for the Laronde CPB can be related to the high degree of saturation maintained in the material, the possible coating of the pyrite grains, and the formation of a thin oxidized layer having a lower porosity (and lower diffusion coefficient) than the bulk CPB. For the Laronde CPB, the addition of binder to the sulphidic tailings appears to have a positive environmental effect on reactive backfill environmental behaviour. (C) 2005 Elsevier Ltd. All rights reserved.