During the past few years, several methods for the measurement of fast neutron flux have been developed in the Cavendish Laboratory. A critical inter-comparison is given in a forthcoming paper by Allen, Livesey and Wilkinson. Some of the methods depend on the counting of protons which are projected by the neutrons from a film of hydrogen-rich material. Polythene is usually employed because its hydrogen content is high and accurately known. If the thickness of the film is comparable with the range of the fastest proton produced by the incident neutrons, it becomes necessary to know the range-energy relation for protons in polythene. This is particularly so in the ‘thick film chamber’ method to be described by Allen and Wilkinson. Another method, the ‘homogeneous ionization chamber’ method to be described by Bretscher and French makes use of the well-known principle that the average density of ionization obtaining in a chamber whose walls have an atomic composition identical with that of the gas which fills the chamber is the same as that in an infinite extent of that gas. This principle itself depends on the identity of mass stopping power of solid and gas of the same atomic composition. The chamber usually used has polythene walls and is filled with ethylene.