This paper addresses a particular type of conflict of laws in international law involving religious freedom. In many European countries, the relationship between state and religion is one of cooperation, which, in some cases, is characterised by the use of treaties. With regard to the Catholic Church, the concordats come into mind, i.e. treaties under international law concluded with the Holy See. They are, for their part, a means of reconciling state law with religious law, namely canon law. Across Europe there are 25 such states, with the European Court of Human Rights recognising these systems. Concordats are considered safe because they are independent of domestic law and difficult to change or end without consent. But it should be noted that the European Convention on Human Rights is also an international treaty. Concordats are formally on the same level as it - neither superior nor inferior. The question now is: what takes precedence when conflicts arise? International law offers no general rule for that. In cases involving concordats (concerning religious marriage, church tax, tax exemption and ecclesiastical ministries), the Court applies the standard fundamental rights test, when applicants allege that their fundamental right has seemingly been violated by the application of a concordat. This test has two entry points for concordats. Firstly, they are cited in in the section 'relevant law', as they may serve as legal basis for the interference. Secondly, they come into play when considering the legitimate aims. One of these is the protection of rights and freedoms of others including religious freedom, even the corporate freedom of the Church. Rights enshrined in a concordat may constitute such rights of others that fall under religious freedom. As case studies show, applying to such a legitimate aim may have the effect that observing the Concordat does not constitute a violation of the Convention. Thus, the court finds a way to balance conflicting rights.