The solid state redox chemistry of microcrystalline salts of the Dawson anion [S2Mo18O62](4-) precipitated with large organic Voltammograms are obtained by mechanically attaching water insoluble heteropolymolybdate salts to a basal-plane pyrolytic graphite electrode which is then placed in aqueous electrolyte media. A considerable number of pH-dependent reduction processes are observed which fall into two categories. (i) Relatively weak responses that correspond to reversible two electron-two proton reduction steps. Despite differences in the mass transport mechanism these five processes are otherwise related to the solution phase voltammetric behaviour of[S2Mo18O62](4-) in acidified organic solvent media and are summarized by the equation [HnS2Mo18O62](x-) + 2e(-) + 2H(+) reversible arrow [Hn+2S2Mo18O62](x-) The observed 60 mV pH(-1) shift of voltammetric potentials is consistent with this mechanism. (ii) At negative potentials and in the presence of an appropriate electrolyte, processes with considerably larger peak currents are shown by electron microprobe analysis of the solid to be accompanied by cation (Cs+, Ba2+) uptake. Voltammetric data confirm that at the potentials where Cs+ and Ba2+ insertion occurs, that the solids are highly reduced. The pH dependence suggests that competition occurs between cation and proton uptake in the highly reduced solid. Therefore the major processes in the presence of a metal ion such as Cs+ are described by the equation (NR(4))(4)[S2Mo18O62] + xCs(+) + yH(+) + (x + y) e(-) reversible arrow (NR(4))(4)CsxHy[S2Mo18O62] The initial voltammetric responses are interpreted in terms of electron transfer and charge neutralization processes that are confined to a region near to the surface of the solid heteropolymolybdate salt, whereas the major processes at more negative potentials that are facilitated by insertion of Cs+ or Ba2+ are associated with changes in the structure of the solid.