Landfill leachate and domestic wastewater were co-treated in batch activated sludge reactors and the ratio of leachate varied from 5 to 20% (v/v). The leachates had a non-biodegradable COD fraction of at least 20%. An increase in leachate adversely affected the co-treatment and it was concluded that the leachate ratio should never exceed 20% of the total wastewater or 50% of the initial COD. The initial Total Kjeldahl Nitrogen (TKN) and the free ammonia level were identified as factors influencing the completion of nitrification.