The space group of alpha'-NaV2O5 turns below T-c = 34 K from Pmmn with all
V sites equivalent, into Fmm2 with three independent vanadium sites per lay
er. This is incompatible with models of charge ordering into V4+ and V5+. O
ur structure determination indicates that the phase transition consists of
a charge ordering with three distinct valence states, formally V4+, V4.5+,
and V5+. Th, singlet formation is not associated with dimerization on the s
pin ladder, but with the formation of spin clusters. Finally, wt: ascribe t
he quadrupling of the c axis to the large polarizability of the V2O5 skelet
on.