A thorough analysis of two corpuses made of oral dialogues gathered in authentic situations and comprising two phases of a dialogue with a machine has allowed the identification of three structuration levels in the structure of a dialogue: the informational one, the interactional one and the syntactic one (concerning the global syntax as well as the local one). These three structuration levels can be described in terms of specific features of pragmatic and, above all, linguistic nature, which appears as absolutely essential to the existence of the dialogue as such.