A kinetic equation for interplanetary cosmic rays is set up with the aid of weak plasma turbulence theory for an idealized radially symmetric model of the interplanetary magnetic field. As a starting point this treatment invokes the Vlasov equation instead of the traditional Fokker-Planck equation. Quasi-linear theory is applied to obtain a momentum diffusion equation for the heliocentric frame of reference which describes the interaction of cosmic rays with convecting magnetic irregularities in the solar wind plasma. Under restricted conditions the well-known equation of solar modulation can be obtained from this kinetic equation.