In fusion plasmas, turbulence-driven heat transport generally dominates both electrons and ions channels. It increases above a threshold in normalized gradient with a rate named stiffness. Heating power modulation experiments provide a direct measurement of the stiffness, as already demonstrated for the electrons, but not for ions so far. In this paper, we report, for the first time, results yielding the ion stiffness deduced from modulation experiments. The experiments were carried out in JET plasmas using Ion Cyclotron Resonance Heating in the 3He scheme. The ion stiffness is larger than that of electrons, but remains moderate and is in agreement with the values yielded by non-linear gyro-kinetic calculations. As a fraction of the electron heating is also modulated in these experiments, a comprehensive analysis of both ion and electron heat transport can be carried out simultaneously, together with an experimental assessment of the ICRH heat sources of the 3He scheme.