As part of a general program of work on opposed piston compression ignition engines operating at high output, it was considered that the scavenging of the engine merited separate attention. Scavenging is of vital importance in the achievement of high output and no information on the operation of opposed piston engines at high boost was available. Further, some idea of the scavenging efficiency was needed for theoretical work proceeding in parallel with the experimental investigation. For the two reasons given above, it was decided that the scavenging of the opposed piston engine operating at elevated pressure should be examined experimentally and an attempt made to relate the measurements to the various theoretical models available.An introduction to scavenging theory will be given, followed by a discussion of experimental methods. A description of the apparatus and a comparison between experimental results and the various theoretical models will complete the paper.