The aim of this study is to evaluate the effectiveness of the NOx charge increase in 2008 on the behavior of NOx emissions from pulp and paper combustion plants, which are in the fee and refund system (regulated plants) in Sweden. In order to evaluate the potential change in NOx emissions behavior, this study used Difference-in-Differences (DID) approach. This involves a ‘control group’ (unregulated plants) and a ‘treatment group’ (regulated plants). The control group consists of smaller plants which are not included in the fee and refund system. The crudest DID assumption is that the average difference in emissions would have stayed constant over time for the control group and the treatment group if the increase of the fee would not have taken place. The results suggest that the NOx charge increase has had negative effects on NOx emission efficiency ("Adoptive NOx Intensity") for some years after 2008. However the reduction in NOx emission efficiency is inconsistent for the period over 2008-13. The difference between the control group and the treatment group jumps up and down in a way that is difficult to explain by the increase of the fee in 2008. This indicates that the DID assumption for the DID estimator may not be fulfilled. It is concluded that the results given these conditions are not reliable to draw any strong conclusions about the possible effect of the increase of the fee in 2008.