Ion energy distribution functions (IEDFs) at the electrodes in single frequency (SF) and dual frequency (DF) radio-frequency discharges in Ar at pressures of 20 and 45 mtorr are measured and calculated. A numerical simulation of the IEDF on the base of a self-consistent particle-in-cell model with Monte Carlo collisions was performed. In addition, a semianalytical model was developed to calculate the IEDF in collisionless and collisional SF and DF plasmas. The IEDF width for the intermediate frequency case was determined from both experimental and theoretical results. The possibility of frequency decoupling is discussed.