[en] A mixture of deuterium and tritium as fuel was introduced for the first time into a tokamak during the Preliminary Tritium Experiment, carried out successfully at JET in November 1991. The main aims were to produce in excess of 1 MW of fusion power, to validate plasma codes under these conditions, to determine tritium retention in plasma-facing components, to establish procedures for tritium removal from components in contact with the plasma, and to demonstrate the technology related to the safe usage of tritium. The experiments were undertaken within limits imposed by restrictions on vessel activation and tritium usage resulting in a tritium inventory of 0.2 g and a maximum number of neutrons of 1.5x10¹⁸. This paper describes the preparations for this experiment with respect to modifications to the vacuum and gas handling system and gives details of materials facing the plasma and conditioning techniques employed. A summary of experimental results is presented together with estimates of the tritium retention in the walls. (orig.)