Aqueous polyiodide solutions were investigated with UV/visible spectroscopy, and equilibria and speciation of solutions relevant to n-WSe2 photoelectrochemistry were determined. At 1 molal or greater iodide concentrations the spectral variation is consistent with assignment of a new 359 nm I-4(2-) absorbance peak and an equilibrium constant of 0.2 associated with I-3(-) + I- reversible arrow I-4(2-):I-, I-3(-), and I-4(2-) are shown to be at significant concentration in these electrolytes. At Light intensities over 10 suns, photoetching and photocorrosion occur in mass transport limited 1 molal polyiodide electrolytes. However, regenerative polyiodide oxidation is sustained in more concentrated electrolytes.