The authors used the direct method of determining sigma /sub n,2n/ on /sup 252/Cf spontaneous fission neutrons. The cross section of the reaction was measured by means of the relative method using activation threshold detectors with known cross sections for neutrons from the Cf spectrum. A figure shows response functions of various threshold reactions with respect to the spectrum of /sup 252/Cf spontaneous fission neutrons. It was necessary for the authors to determine which of the cross sections contained the larger systematic error.