On the Joint European Torus ( JET ) , a key source of calibrated electron temperature T e profiles is from the measurements of the full electron cyclotron emission ( ECE ) spectrum made by a Fourier transform spectrometer ( FTS ) . It is absolutely calibrated by using a calibration source inside the vacuum vessel. High spatial and temporal resolution ECE T e profiles are obtained using a 96-channel heterodyne instrument which is cross-calibrated on each JET pulse against the data from the FTS system. Residual systematic frequency dependent errors at the 5%–10% level can then be evaluated and corrected for, using specific discharges in which the toroidal field is varied while keeping the shape of the T e profile constant. This improvement in the calibration method has been systematically applied at JET for the first time improving both the smoothness and the symmetry of the T e profiles. The consequences of this improvement are discussed. In addition, it is shown that no deviation occurs in the FTS calibration for more than eight years, which is relevant for the International Thermonuclear Experimental Reactor.