The effects of localized absorption processes on mode conversion in the various RF heating schemes for fusion plasmas are analysed. An asymptotic separation technique separates local and asymptotic behaviour and leads to an integral equation through use of a fourth-order Green's function. Detailed calculations are presented for the deuterium ion cyclotron harmonic case with a hydrogen impurity ranging from impurities dominant to impurities absent. It is shown that as k∥ increases, mode conversion ceases and only ion cyclotron absorption occurs. Details of the E+ field, which has a minimum at resonance, and rotational transform effects are presented. The technique is applied to the other mode conversion processes (Alfvén resonance, electron cyclotron harmonic, incident slow wave) but no numerical results are given.