Is the degree of ionization always relevant for ambipolar diffusion in interstellar clouds

The well-known result that ambipolar diffusion proceeds faster at smaller degrees of ionization is reexamined. It is concluded that, in interstellar clouds of moderate density, the degree of ionization is not relevant to ambipolar diffusion. In much denser clouds in which gravitational forces can dominate, the degree of ionization does not uniquely determine the rate at which ambipolar diffusion progresses; the specification of two additional dimensionless parameters is necessary. The underlying physics is discussed.