This paper is concerned with the propulsion of an isolated slug of liquid along a pipeline by an expanding gas. The particular case in mind is when a body of liquid accumulates behind a closed valve in a piping system carrying a wet or condensing gas, and that valve is suddenly opened to expose the slug of liquid to a high-pressure gas. The slug will be driven along the pipeline by an expansion wave which weakens as the slug accelerates, whilst its motion will be resisted by friction and by the compression wave ahead of it. This situation has been analysed and compared with slug velocities measured in a 50 mm pipe. Also presented in this paper are measurements of the considerable forces generated by the slug as it impinges onto an orifice plate installed in the pipe.