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Three pairs of fission counters (each pair one 235U and one 238U) are used at the Joint European Torus to determine the time dependence of the neutron production. In order to determine the absolute value of the neutron flux at the detector location it is necessary to know the neutron detection efficiency of the counter assemblies. This was measured using monoenergetic neutrons (at 2.5 and 14 MeV) and Cf and Am/Be sources. The fraction of fissions detected was determined by extrapolation of the pulse‐height spectrum to zero pulse height. The calculation of efficiency was made with the Monte‐Carlo neutron transport code MORSE. It was found that the detailed structure of the counter significantly affected the calculated efficiency and that the thermal cross‐section values of the DLC37F nuclear data library had to be replaced with room‐temperature values. The mean difference between calculation and experiment is (5.5±6.3)%.