This study examined the impact of mandatory corporate social responsibility (CSR) disclosure, CSR assurance and the reputation of assurance providers (accounting firms) on the cost of debt capital. Our difference-in-difference research design in conjunction with univariate and multiple regression analysis was assessed using a large sample of firms listed on the Taiwan Stock Exchange and the Taipei Exchange. Our empirical results revealed that mandatory CSR assurance on CSR disclosure provided by accounting firms tended to reduce the cost of debt capital. However, contrary to expectations, the reputation of the accounting firm (Big 4 accounting firms vs. non-Big 4 accounting firms) tasked with providing CSR assurance did not have a significant effect on the cost of debt capital. These results have implications for firms seeking an assurance provider as well as for Big 4 accounting firms. These results also provide specific evidence relevant to government agencies seeking to update policies and extend the scope of mandatory CSR assurance to other environmentally sensitive industries.