In this article, the authors discuss the Portuguese banking sector solidarity contribution (BSSC) in light of the ECJ’s decision in Cofidis v. Autoridade Tributária e Aduaneira (Case C-340/22). It explores how specific institutions (banks), in issuing certain financial instruments, generate an unequal tax treatment that impacts the freedom of establishment of branches. The article looks at the potential sources of the difference in tax treatment discussed in the ECJ decision from a legal and financial perspective.