Selenide and polyselenide UV-vis absorption spectroscopic characteristics were investigated for aqueous polyselenide solutions. From these observations equilibria interrelationships between OH-, H+, H2Se, H2Se-, Se-=, Se-2(=), Se-3(=), and Se-4(=) were probed. The standard cell potential for the reduction of 1/2 Se-2(=) to Se-= was determined to be -0.716 V. The equilibrium reactions of Se-3(=) with Se-= to 2 Se-2(=) and 2 Se-4(=) with Se-= to 3 Se-3(=), respectively, have pK(23) = -0.7 (+/-0.1) and pK(34) -4.0 (+/-0.1) in 0.5 to 3M aqueous electrolytes. Consistent with these equilibria, the free energy of formation for Se-2(=), Se-3(=), and Se-4(=), have been determined as 121, 115, and 121 kJ/mol, respectively Polarization studies of these polyselenide solutions demonstrated Pt and CoSe as efficient catalytic surfaces for the oxidation and reduction of selenide species.