This article considers the European Court of Justice (ECJ) decision of 3 April 2008 in Derouin, with emphasis on the importance of the ruling for social security contributions, the 1968 France-UK tax treaty (the Treaty) and Regulation No. 1408/71 (the Regulation).In this respect, it should be noted that the Treaty and the Regulation follow different rules regarding the allocation of taxation rights and the designation of the applicable national social security legislation, respectively.