The paper deals with a circularly polarized electromagnetic wave propagating along a non-uniform magnetic field. A general wave equation describing the propagation of the wave is obtained, and by means of this equation the physical processes which occur when the wave frequency coincides with the cyclotron frequency and which lead to absorption of the oscillations are analysed. Corrections to the absorption coefficient to allow for the Doppler effect, particle acceleration and the non-linear character of the variatio n in an unperturbed magnetic field are found. It is shown that in calculating the coefficient of absorption use of the adiabatic wave equation leads to the same result as direct consideration of the resonance interaction.