This article studies the price relationships between EU emissions allowances (EUAs) - valid under the EU Emissions Trading Scheme (EU ETS) - and secondary Certified Emissions Reductions (sCERs) - established from primary CERs generated through the Kyoto Protocol's Clean Development Mechanism (CDM). Given the price differences between EUAs and sCERs, financial and industrial operators may benefit from arbitrage strategies by buying sCERs and selling EUAs (i.e. selling the EUA-sCER spread) to cover their compliance position between these two assets, as industrial operators are allowed to use sCERs towards compliance with their emissions cap within the European system up to 13.4%. Our central results show that the spread is mainly driven by EUA prices and market microstructure variables and less importantly, as we would expect, by emissions-related fundamental drivers. This might be justified by the fact that the EU ETS remains the greatest source of CER demand to date.