A new model is described for cation transport in glass, in which below T-g a dynamic equilibrium exists between 'open' and 'closed' sites. Cations migrate by hopping into a relatively small number of open sites, which must be just the right size for them to occupy. This model accounts plausibly for the apparent constancy of activation volumes (-RT d lnsigma/dP) in single cation glasses, since the activation volumes are identified with the volumes of the open sites. The effect of increasing the external pressure is to close off some of these sites, thus reducing the number of ions which are free to move. This new approach draws emphasis away from the nature of 'occupied' to 'unoccupied' sites in glass; in this sense it is consistent with the 'dynamic structure model' proposed by Bunde et al.