Ion energy distributions (IEDs) at an r.f.-biased electrode in an inductively coupled plasma-etching chamber were measured by an r.f. floating ion energy analyzer (IEA) with a quadrupole mass spectrometer (QMS). For Ar-O-2 plasma, the IEDs of Ar+ and O-2(+) could be qualitatively explained by a model calculation that included charge exchange collisions in the sheath. On the other hand, the IED of O+ could not be explained by the model. However, if we assume that a fast O+ was generated from a fast O-2(+) in the sheath, the measured O+ IED can be qualitatively explained. For Ar-C3F8 plasma, CF+ and CF2+ IEDs also could not be explained by the simple charge exchange model, but if we assume that both a fast CF+ and a fast CF2+ were generated from fast CF3+ in the sheath, the measured IEDs can be qualitatively explained.