The aim of this thesis is to study the influence of the notion of "libre disponibilité des droits" on the choice of law process. This notion tells how pleading and proof of foreign law must be done. It serves to determine the conflict rule's binding force towards the judge and the parties. The first part analyses the notion of "libre disponibilité des droits". The second part shows its incidence on the judge's role. If the parties' rights are "disponibles", the judge is not bound to apply ex officio the foreign law, unless the conflict rule emanates from an international treaty. But, according to the French civil law; the judge's mission is to apply the rule of law in all cases. At this point, the notion of "libre disponibilité" should not be taken into account. The third part of this thesis shows that the criterium still plays a fundamental role. The parties, whose rights are "disponibles" can impeach the judge to apply the conflict rule that designates a foreign law. Therefore, it is suggested that the judge should always be bound to apply ex officio the conflict rule, unless the parties have expressly asked him not to do so. To this effect, they must be able to dispose freely of their rights.