The tightening of the European Union climate and energy policy, directed to raising the resilience and effectiveness of the European Union Emissions Trading System, may influence on companies' economic performance. Polish energy companies, which mainly use coal in the energy production process, are particularly worried about the potential negative consequences associated with the implementation of new CO2 emission reduction targets and their participation in the European Union Emissions Trading System. This paper analyzes the link between economic performance and environmental performance of Polish energy companies, covered by the European Union Emissions Trading System in the years 2013-2017. This analysis may focus the attention of the European Union’s and Polish institutions responsible for developing the climate and energy policies on positive and negative consequences concerning the low-carbon transformation of the Polish energy sector. Two indicators of environmental performance: carbon intensity and the ratio of carbon dioxide emissions over the allowances, as well as chosen financial ratios: return on assets, return on investment, return on sales, asset turnover ratio are calculated for these companies. A Spearman’s rank correlation coefficient is used in order to analyze the relationship between these environmental and financial variables. Presented empirical results highlight that the situation of Polish enterprises worsened in the 3rd trading period, as the number of allocated allowances in case of all energy companies was insufficient to cover their own CO2 emission. A negative direction of the correlation dependency can be observed in the years 2013-2017 only between asset turnover ratio and return on assets, and the CO2 emission intensity.