The concentration of deuterium in JET plasmas, expressed as a fraction of the electron concentration, has been determined using eight different methods, four of which involve neutron detection. The results from these various methods are found to be consistent within their experimental errors. The ratio nD /ne, measured at the moment of peak neutron emission strength, is found to lie in the range from nearly unity, for discharges into which deuterium pellets are injected, down to values of 0.4 or less for some of the highest performance discharges. Thisfinding is based on the analysis of discharges run in 1988, when the plasma-facing components within the vacuum vessel were of carbon or were carbon coated.