Room temperature molten salt systems based on methyl-hexyl-imidazolium iodide (MHImI) have been used to scrutinize the performance characteristics, the stability and the mass-transfer effects in a photoelectrochemical regenerative device, as the latter is influenced and can even be limited by local concentration and mass-transport of the electroactive redox mediator species in the electrolyte phase. These salts appear to afford particular advantages over organic liquids as solvents for solar cell electrolytes. Cell performance showed outstanding stability, with an estimated sensitizer turnover in excess of 50 million. An investigation has been carried out on the physical-electrochemical properties of MHImI and its mixtures with organic solvents such as n-methyl-oxazolidinone, acetonitrile and with other lower viscosity molten salts such as methyl-butyl-imidazolium triflate. The repercussions of these properties on solar cells is described experimentally by the performance of practical application devices. Simulation models of mass transport in the nanocrystalline solar cell help illustrate operational aspects such as concentration profiles, limiting currents, anticipated mass-transfer overpotential as a function of current density, and they help to make projections as to how the properties of molten salt electrolytes can be better exploited toward this practical end.