The time-dependence of the 2.5MeV neutron emission from JET is reliably measured using fission chambers. The absolute calibration of these chambers is required to an accuracy of 10%, or better, for a range of intensities that may cover 6 or more decades. At JET, this calibration is now achieved by use of activation techniques, the most convenient of which involves fissionable materials (thorium and uranium) and delayed neutron counting. Because delayed neutron counting is unfamiliar in the fusion community, particular care was taken to obtain confirmation of the results based on this method by comparison with measurements made using the conventional activation procedure (involving indium, nickel and zinc as target materials). As the activation measurements can be influenced appreciably by the weak emission of 14MeV neutrons, this contribution was measured separately using high threshold energy activation reactions (in copper and silicon). Neutron transport calculations are employed to relate the measured local fluences of both 2.5MeV and 14MeV neutrons to the total yields from the plasma. Absolute calibration accuracies of 6% and 8% are claimed for 2.1MeV and 14MeV neutron yields, respectively; the accuracy of the 14MeV to 2.5MeV yield ratios is 6%.