Nitrate and perchlorate were identified as significant water contaminants all over the world. This study aims at evaluating the performances of the heterotrophic-autotrophic sequential denitrification process for reductive nitrate and perchlorate removal from drinking water. The reduced nitrate concentration in the heterotrophic reactor increased with increasing methanol concentrations and the remaining nitrate/nitrite was further removed in the following autotrophic denitrifying process. The performances of the sequential process were studied under varying nitrate loads of 0.6 -1.2 g NO3- -N/(Ld) at a fixed hydraulic retention time of 2 h. The C/N ratio in the heterotrophic reactor varied between 1.24 and 2.77 throughout the study. Nitrate and perchlorate reduced completely with maximum initial concentrations of 100 mg NO3 -N/L and 1000 mu g/L, respectively. The maximum denitrification rate for the heterotrophic reactor was 2.4 g NO3 -N/(Ld) when the bioreactor was fed with 100 mgNO(3)(-)-N/L and 277 mg/L methanol. For the autotrophic reactor, the highest denitrification rate was 0.86 mgNO(3)(-)-N/(Ld) in the first period when the heterotrophic reactor performance was low. Perchlorate reduction was initiated in the heterotrophic reactor, but completed in the following autotrophic process. Effluent sulphate concentration was below the drinking water standard level of 250 mg/L and pH was in the neutral level.