ICRF heating of impurity-free tokamak plasmas is studied in DIVA. The frequency is fixed at 25 MHz, which is a second-harmonic cyclotron frequency of deuteron with a toroidal magnetic field of 16.4 kG. The toroidal field BT and the ratio of proton-to-deuteron density p are varied over ranges of 12 ⩽ BT ⩽ 19 kG and 2 ⪅ p ⪅ 40%, respectively. The most favourable ion heating is obtained at BT = 18 kG and p = 5 ∼ 10%. Ion temperature increases from 200 to 300 eV were measured by charge-exchange neutral analysis and also by spectroscopy, where 100 kW RF net power were applied. The results are consistent with mode-conversion theory.