The radical rise of global temperatures has put high pressure on the environment, leading to societal pressure towards politicians to reverse the trend. In light of this environmental crisis, economists all over the world agree that carbon taxes are the most cost-effective instrument for reducing carbon emissions. This study uses difference-in-differences analysis to examine the treatment effect of a carbon tax implementation in Denmark, Finland, Norway, and Sweden. Panel data from 24 OECD countries, between 1978 and 2004, is used throughout this study. This study has not found support that different levels of democracy and GDP per capita affect tax efficiency. Moreover, higher levels of urbanisation are seen to undermine the efficiency of a carbon tax, however, the result may be affected by confounding bias. Lastly, tax rate and tax coverage are both factors that seem to affect carbon tax efficiency.