Estimates of the magnitude of CO2 emissions reduction brought about by an intervention in the energy system are important because they signal which interventions are the most potent in terms of climate change mitigation. Yet quantifying emissions changes is not trivial because interventions act on the margin of the energy system, rather than acting on all components of the whole energy system equally. Therefore, in order to accurately attribute outcomes to interventions, the specific energy system changes precipitated by the intervention should be estimated, along with the corresponding change in emissions. This paper builds on previous research in this regard estimating short-run marginal emissions factors in national electricity systems. It presents the concept of the long-run marginal emissions factor (LR-MEF), and builds and applies a new electricity system model to study the problem. For the British electricity system it is found that the average LR-MEF is approximately 0.26-0.53 kg CO2/kWh for the coming decade, but this reduces to approximately zero by 2035 and onwards as the system decarbonises. Furthermore, it is found that the LR-MEF can diverge very significantly from the short-run. This highlights the state of flux of the British electricity system and the importance of taking structural changes in the electricity system into account when attributing emissions reduction to interventions. (C) 2014 The Author. Published by Elsevier Ltd.