Microcavities have been formed at the tip of platinum microelectrodes and packed with [Os(bpy)(2) (H(2)tzt) Cl]PF6, where bpy is 2,2'-bipyridyl and H(2)tzt is 3,6-bis(4-pyridyl)-dihydro-1,2,4,5-tetrazine. These solid deposits exhibit well defined electrochemical responses associated with the Os2+/3+ redox couple where the identity of the electrolyte anion is sodium perchlorate, bromide, chloride, iodide, fluoride or nitrate. Scanning electron microscopy of deposits on planar electrodes reveals that voltammetric cycling triggers morphological changes that depend on the identity of the electrolyte anion. The formal potential of the Os2+/3+ redox process ranqgs from 0.225 +/- 0.015 V (perchlorate) to 0.390 +/- 0.010 V (fluoride) and depends approximately linearly on the hydration energy of the anion. This result suggests that weakly hydrated anions are thermodynamically easier to incorporate within the solid deposit in response to redox switching, i.e., the anion desolvation energy influences the energetics of redox switching. Cyclic voltammetry has been used to determine the apparent charge transport diffusion coefficient, D-CT, describing homogeneous charge transport through the deposit. The rates of charge transport depend significantly on the identity and concentration of the supporting electrolyte ranging from a minimum of 3.1 +/- 0.5 x 10(-12) cm(2) s(-1) in 0.1 M NaF to a maximum of 1.1 +/- 0.1 X 10(-10) cm(2) s(-1) in 1.0 M NaClO4. In NaClO4 supporting electrolyte, D-CT is independent of the electrolyte concentration from 0.1 to 1.0 M suggesting that electron self-exchange between adjacent redox centres limits the overall rate of charge transport through the solid. In contrast, in NaBr solutions D-CT is sensitive to the electrolyte concentration increasing from 0.2 +/- 0.01 to 7.9 +/- 0. 1 X 10(-11) cm(2) s(-1) on going from 0. 1 to 1.0 M suggesting that the availability of charge compensating counterions within the solid limits the rate of charge transport. The free energy of at least partially desolvating charge compensating counterions prior to their incorporation within the oxidised deposit appears to control both the energetics and dynamics of charge transport.