The aim of the study is to investigate the effect of environmental expenditures on environmental regulations and practices of companies operating in different sectors included in the BIST Sustainability Index in Turkey, on the financial performance of companies. The data obtained from the financial and sustainability reports of 18 companies, which are included in the BIST Sustainability Index in Turkey, operating in different sectors and selected randomly between the years 2015-2020, were used. Financial data were obtained from the Public Disclosure Platform and sustainability reports were obtained from the websites of the companies, and these data were measured with the help of system-GMM data analysis. In this study, the effect of companies' environmental expenditures on the return on capital employed (ROCE) is investigated; Environmental expenditures were determined as independent variable, return on capital used as dependent variable, and firm size as control variable. As a result of the analysis made with the System-GMM method, a negative and significant relationship was determined between the environmental costs and the return on the capital used, and it was concluded that a 1% increase in the environmental costs would reduce the return on the capital used by 0.08%. In the study; It is thought that the companies operating in Turkey and reporting this using environmental accounting will contribute to both the literature and the widespread use of environmental accounting by companies, thanks to the findings on the relationship between environmental costs and profitability levels. During the execution of the study, it was observed that in some of the sustainability reports published by companies included in the BIST Sustainability Index, environmental expenditure amounts were not specified at all or were stated in different currencies. This situation limits sample selection and determination in empirical studies.