I examine in the period 2008-2011 the efficiency of four carbon dioxide (CO2) emission allowance futures traded in the Intercontinental Exchange (ICE). To this end, I assess the profitability of trading strategies based on simple technical analysis rules and naive forecasts. The results from 2010 onwards are consistent with weak market efficiency. In turn, this finding suggests that the European carbon market is gradually attaining a state of maturity. (C) 2012 Elsevier Ltd. All rights reserved.