Ion cyclotron resonance heating and nonadiabatic trapping of the hydrogen ions are investigated using a magnetic mirror machine with a mirror ratio of 2. A high energy (150–450 eV) and low intensity (500 µA) hydrogen ion beam is injected along the magnetic lines of force and subjected to ion cyclotron resonance heating by a constant frequency (4.5 Mc/s) r.f. electric field at the midplane of the magnetic mirrors. Nonadiabatic trapping of the atomic and the molecular ions is observed distinctly. More than fifty percent of the injected atomic and molecular ions is reflected by the mirror field, the reflected ions having energies of 300–900 eV.