This paper attempts to develop a theory of the role of ambipolar phenomena in the mechanism of the post-zero current in high-current vacuum circuit breakers on a more general and more accurate basis that can be found in the literature. Several cases of interest are compared and discussed and their probable range of validity established. Whereas the theory presented in this paper is probably adequate when the density of the neutral gas is low (mean free path comparable to electrode separation), it should probably be combined with other recognized mechanisms, notably gross gas motions and turbulence, when the pressure is higher during the afterglow. No numerical check is presented here, most of the basic data still being much too uncertain.