Two series of JET shots, one on hot ion shots and one on hot electron shots, have been analysed in order to improve the understanding of how the temperature ratio Te/Ti influences the performance. Comparisons are also made with ITER simulations which are in the hot electron regime. The temperature ratio has been varied between different shots, in partly interpretative simulations of one shot and locally. The effect of the variation of the temperature ratio depends strongly on how it is done but can also be different in different types of shots. In general, however, an increased electron heating deteriorates confinement rather strongly. This is the situation for increasing the alpha power in ITER. ITER simulations show, however, that the effect is not dramatic.