The paper considers the conflict of interests in representation in terms of the individual-psychological phenomenon as an intrapersonal conflict of interest. The latter is due to the different positions held by an individual. Given that a representative is not himself the bearer of interests which he must protect as a representative, it may quickly happen that he gives priority to his own interests or those of a third party over the interests of the person represented, thus finding himself in a conflict of interests. Due to the principle of abstractness in the law of agency, the conduct of a representative who is in conflict of interests as a representative’s breach of an obligation arising from the internal relationship does not, as a rule, have an effect on the validity of the representative transaction in the external relationship with the contracting party. The main subject of the study will be situations in which the conflict of interest exceptionally has an impact on the validity of the concluded representative transaction in the external relationship. There are special legal provisions in Slovenian law which regulate the impact of conflict of interest on the validity of a representative transaction in individual cases, i.e. rules which, in order to prevent conflict of interest, restrict the performance of the representative in various ways. Since there are no explicit legal provisions in Slovenian law governing the impact of conflicts of interest on the validity of a representative transaction at a general level of civil law, the aim of this paper is to define the criteria for the latter. In this context, the rules governing the validity of self-contracting and double mandate, the rules of collusion between the representative and the contracting party, as well as the rules regarding the abuse of agency are outlined. The assumptions under which conflict of interest in these cases can have an impact on the validity of representative transaction are analyzed and the legal consequences which such conduct of the representative in relation to the person represented and the contracting party may have were elaborated. In terms of comparative law, the paper deals with the regulations in the German and Swiss legal systems and some rules at the transnational level. With regard to the latter, the paper presents the relevant provisions of EU law, the provisions of PECL, DCFR and PICC.