In this CFE opinion statement, the CFE ECJ Task Force comments on the ECJ decision in MK v. Autoridade Tributária e Aduaneira (Case C-388/19) of 18 March 2021. The Court confirmed its previous case law and held that the Portuguese (optional) regime for taxation of capital gains from immovable property of non-residents was contrary to the free movement of capital under article 63 of the TFEU since non-residents were taxed less favourably than residents.