In the present paper the effect of a sinusoidal modulation of an electromagnetic field on the invariance of the magnetic moment is studied. Such a generalized invariant plays an important role in problems concerning the motion of charged particles in the non-uniform magnetic field of the magnetosphere or the solar wind. In order to find an adiabatic invariant J, a canonical transformation is introduced, and J is expanded in an asymptotic series in the relative modulation amplitude. We are studying the first and second order terms of this expansion. It is further shown that the curves J = constant closely fit the results obtained by a numerical integration of the system of differential equations governing the motion of the particles.