We investigate the interaction of debt covenants and tax accounting on the adoption of Financial Interpretation No. 48 (FIN 48). We examine how firms respond to the potential tightening of covenant slack upon FIN 48 adoption and whether these actions are penalized by creditors and anticipated by equity markets. We find that upon FIN 48 adoption, the majority of sample corporate borrowers increase their tax reserves and reduce equity. Firms close to debt covenant violation were even more likely to increase tax reserves upon FIN 48 adoption; however, the size of the adjustment was relatively smaller, suggesting that the FIN 48 standards limited, but did not eliminate, firms use of discretion in reporting uncertain tax positions to avoid costly covenant violations. For firms near net worth debt covenant violation, the act of decreasing equity upon FIN 48 adoption imposes real economic costs, as the average cost of debt increased by 43 basis points. Finally, we extend prior research on the market response to FIN 48 by showing how the market response to FIN 48 adoption is a function of debt covenant slack and tax aggressiveness. Specifically, the cumulative abnormal return at the FIN 48 exposure draft release date is negative only for tax aggressive firms that are close to debt covenant violation.