The use of a commercial G-M counter as a neutron insensitive device in mixed neutron/photon fields was explored experimentally. The counter was operated in the proportional region, and the event size spectra were measured and analysed. Pulse height spectra were collected over a dynamic range of 104. Considerable experimental difficulties were encountered because of the steep slope of the relationship between pulse height and collection voltage in the proportional region. Values of k(u) for monoenergetic neutrons of 0.57 and 1.5 MeV were obtained from an analysis of the pulse height spectra. They are compared with data available from other techniques. Theoretical considerations are given to explain qualitatively the spectra caused by the nuclear reactions of the neutrons in the counter gas and wall.