  We show that the proposed zig--zag charge--ordering model for alpha'--NaV2O5
is incompatible with a single transition that induces both charge--ordering and
spin gap. We introduce a two--band model for alpha'--NaV2O5 within which the
simultaneous charge--spin order transition is driven principally by
electron-phonon interactions. The spin gap is due to the formation of local
dimers, thus explaining the weak magnetic effect on this transition.
