Cryospray mass spectrometry (CSI-MS) has been used to probe the mechanism of self-assembly of polyoxometalate clusters in solution. By using CSI-MS and electronic absorbance spectroscopy it was possible to monitor in real-time the self-assembly of polymeric chains based on [Ag2Mo8O26](2-)(n) building blocks. The role of the Ag-I ion in the solution state rearrangement of molybdenum Lindqvist ({Mo-6}) into the silver-linked beta-octamolybdate ({Mo-8}) structure ((n-C4H9)(4)N)(2n)[Ag2Mo8O26](n) (1) is revealed in unprecedented detail. A monoanionic series, in particular [AgMomO3m+1](-) where m = 2 to 4, and series involving mixed oxidation state polyoxomolybdate species, which illustrate the in-solution formation of the (Ag{Mo-8}Ag) building blocks, have been observed. CSI-MS detection of species with increasing metal nuclearity concomitant with increasing organic cation contribution supports the hypothesis that the organic cations used in the synthesis play an important structure-directing role in polyoxometalate (POM) growth in solution. A real-time decrease in [{Mo-6}] and associated increase in [{Mo-8}] have been observed using CSI-MS and electronic absorbance spectroscopy, and the rate Of {Mo-6} interconversion to ({Mo-8}) was found to decrease on increasing the size of the countercation. This result can be attributed to the steric bulk of larger organic groups hindering {Mo-6} to {Mo-6} rearrangement and hindering the contact between silver cations and molybdenum anions.