Even though he is mainly known for his concept of constituent power, Sieyès was one of the first constitutional theorists to ask for a guardian of the constitution which closely resembles contemporary constitutional courts. This article reconstructs the main tenets of his proposal, puts them in the larger context of his constitutional theory and then assesses the constitutional nature and functions of this institution. The judgment is mixed: as an organ, Sieyès’ constitutional jury is a hybrid institution, neither a real (...) third chamber nor a full-fledged constitutional court; however, its functions not only are a clear anticipation of the control of constitutionality, but are also intended to tame constituent power and to protect the rights of man in case of legal gaps. (shrink)