We apply an index decomposition analysis to investigate the main drivers of CO2 emissions in the electricity generation sector in Spain over the period 1991–2017. We quantify the impact of five different effects: carbonization, transformation, fossil intensity, electricity intensity, and production effects. Four subperiods are identified. The relevance of the drivers changed over these subperiods. The fossil intensity, electricity intensity, and production effects played an important role in the increase in emissions during the first half of the period, and particularly from 1999 to 2005. In contrast, the carbonization and fossil intensity effects were the dominant drivers of emissions reductions between 2006 and 2010. The research allows an assessment of the impact of different measures on emissions by considering their influence on the different effects, and suggests which sets of measures could be more effective in reducing emissions. Peer reviewed