The mass-resolved ion energy distribution (IED) at the grounded electrode has been determined in a 13.56-MHz parallel-plate plasma in argon. The IED is determined of Ar+, Ar2+, Ar2+, ArH+, H3O+, and H3+ for several plasma conditions. At pressures higher than 10 mTorr, collisions in the sheath become important. The IED of Ar+ is particularly defined by charge exchange collisions in the sheath while the IED of the other ions shows only features generated by elastic scattering. This is confirmed by Monte Carlo simulations. The measurements clearly show the necessity of simultaneous mass and energy separation, rather than the nonmass-resolved IED reported in the literature.