Treatment of sulphite mill condensates containing acetate and ammonia was studied using aerobic thermophilic bacteria at 65°C. A separate treatment at this relatively high temperature could save energy and decrease water consumption in the process. Experiments were carried out in packed bed bioreactors (vol. 0.1–0.3 dm3) using media based either on condensate or on ammonium acetate. Diatomaceous earth was superior to the other carrier materials tested (red brick, activated carbon, pumice). At 90% acetate removal, the acetate consumption rates were ca 0.7 and 0.5 g dm−3 h−1 on ammonium acetate (acetate 5 g dm−3) and dilute condensate media (acetate 2 g dm−3), respectively. The corresponding dilution rates were 0.15 and 0.38 h−1.