This experiment is concerned with a method of determining cross-sections of (n,2n) reactions, taking advantage of the fact that the residual nucleus in most cases is a positron emitter which permits detection by coincidence counting. In this paper, the N14(n,2nN13, Cl35(n,2n)Cl34, and F19(n,2n)F18 reactions are considered. These reactions have Q-values of -10.9, -13.2, and -10.3 mev respectively, and may he studied with the 14.1 mev neutrons which are produced by the T3(d,n)He4 reaction by the Rice Institute Cockroft-Walton Accelerator. The N14, Cl34, and F18 nuclei decay by positron emission with half-lives of 10.2 min, 33 min, and 112 min respectively. The procedure of the experiment involves irradiating the substance under consideration for a measured time with a known flux of 14.1 mev neutrons, and then observing the coincidences due to the annihilation photons with an appropriate detecting system, the difficulties involved in calculating solid angles, efficiencies of the counters, etc., are avoided by calibrating the system in terms of a Na22 solution, which is absolutely calibrated previous to the experiment, the number of coincidence counts produced by the radioactive substance over a measured period of time is then easily related to the cross-section for the reaction.