A critical point in the determination of the ITER performance is to understand the behaviour of heavy impurity density profiles, in particular if in any plasma condition metallic impurity accumulation has to be expected, and how it can be controlled. In JET, heavy impurities have been found to behave differently in low collisionality H-mode discharges depending on the electron power deposition profiles and on the predominant RF heating channel. If RF heating is applied in Mode Conversion (MC, with ~20% 3He) to heat electrons, Nickel density is found to be flat, while with ICRH in Minority Heating (MH, with ~8% 3He, to heat ions) Nickel peaks. In this paper these results are presented and discussed within the ITG/EM stability theory. To analyse the impurity behaviour the technique of nickel Laser Blow Off (LBO) injection has been used.