The effect of the aeration of a landfill in its humic phase on the leaching potential of deposited metals has been investigated. Aeration increased the cation-exchange capacity, but decreased the buffering capacity and the metal–binding capacity. Leachates from the aerated landfill contained less than half the amount of aluminium, iron and molybdenum and more than double the amount of calcium, magnesium, sulfur and zinc of leachates derived from an identical, but anaerobic, landfill. The leachate from the aerated landfill was capable of extracting metals from the original landfill when recirculated. Leachate from the original landfill treated with leachate from the aerated landfill contained more than twice as much aluminium, boron, cadmium, chromium, copper, iron, lead, magnesium, nickel, phosphorus, potassium, sulfur and zinc than leachate from the identical anaerobic landfill. It was concluded that the mobility of deposited metals in landfills will change over time as the ageing of the landfills progresses and conditions become more oxidized.