Spatial profiles of the neutron emission from deuterium plasmas are routinely obtained at the Joint European Torus (JET) using the line-integrated signals measured with a multi-channel instrument. It is shown that the manner in which these profiles relax following the termination of strong heating with Neutral Beam Injection (NBI) permits the local thermal diffusivity (ci) to be obtained with an accuracy of about 20%. The profiles of ci for small minor radius (r/a < 0.6) were found to be fiat and to take values between 0.3 and 1.1 m2s­1 for H-mode plasmas with plasma current Ip = 3.1MA and toroidal field BT = 2.3T. The value of ci was smallest for Zeff = 2.2 and increased weakly with increasing Zeff. The predictions of neoclassical theory are much smaller than the experimental values for low Zeff but increase rapidly until they become comparable with the experimental values at the highest Zeff obtained.