At JET, experiments have been conducted in order to optimize the performance of ion cyclotron radio-frequency (ICRF) heating in the presence of the newly installed ITER-like wall. The ICRF was tuned to the cyclotron frequency of minority hydrogen (H), which is also the 2nd harmonic of the cyclotron frequency of deuterium (D). The relative H concentration was varied from a few percent up to about 25 percent in a series of plasma discharges and the neutron time-of-flight spectrometer TOFOR was used to measure the energy spectrum of neutrons from the D(D,n)3He reaction during these discharges. It could be seen that the signal from ICRF accelerated deuterons depended strongly on H concentration. This observation indicates that the ICRF power absorbed by D at the 2nd harmonic cyclotron resonance increases with decreasing H concentration. This is in qualitative agreement with theoretical expectations obtained from the plasma wave equation solver TOMCAT.