The sacrament of marriage between a Catholic and an Orthodox member of the faithful is treated in canon law as a mixed marriage. However, owing to the doctrinal affinity between the two Churches, marriage is also treated differently than other possible mixed marriages. This, however, does not eliminate the need for compliance with the legal regulations when obtaining the required permission and possibly a dispensation from canonical form. The article focuses on the reasons and possible canonical and pastoral issues that may pose problems in such marriages. Therefore, with the concept of mixed marriage and legal requirements presented, the dangers related to mixed marriage are also indicated – all kinds of spiritual threats to the Catholic party that may be experienced in such a marriage. The potential difficulties include divergent notions of marriage, excessive attachment to one’s own Church, and the danger of religious indifferentism, impediments to worship, difficulties in the religious education of children. The permission of the local ordinary, as prescribed by law, should meet conditions that will help the competent authority to decide a specific case. The article also lists the most common situations that priests may encounter in the case of Catholic-Orthodox marriages, for example: difficulty ascertaining that a person is baptised or single, aversion to the institution of promises on the Catholic or non-Catholic side, expression of the desire to join the Church of the other party.