A new non-contaminating piston-type compressor is described. A highly flexible rubber diaphragm behind the piston acts as a gas-tight gland and also separates the lubricated parts which drive and guide the piston from the space swept out by the piston itself, in which there is no fluid lubricant. The diaphragm is specially protected against pressure differences by suitable connexions.The experimental 2-stage model constructed, which has been used in the liquefaction of helium, is based on a 2-cylinder motor-cycle engine. Details are given of its performance.