This article discusses a situation in which international conventions that include norms of private law are applicable in situations other than those stipulated by norms that define their scope of application (ex proprio vigore). This involves various cases in which the domestic, EU or international legislator refers to the provisions of a given international convention (in a different international convention) and also cases in which provisions of an international convention are applied by virtue of the will of the parties to a civil law relationship. The author analyses individual situations searching for an answer to the question about the nature of provisions used and thus – adequate rules for their interpretation or for filling of gaps. In some situations, they retain their international law nature, while in others they become part of the domestic or EU law or a certain standard form contract which profiles the civil law relation.