The [Os(NH3)(5)(OH2)](2+) complex in aqueous acid has been widely regarded as short-lived on the basis of results reported in an early pioneering electrochemical study.(1) More recently, a much longer-lived Os(II) species generated by reduction of [Os(NH3)(5)(OH2)](3+) but of uncertain composition has been reported.(6) In the present study the lifetime of [Os(NH3)(5)(OH2)](2+) in aqueous acid was determined by steady-state voltammetry at a carbon microelectrode and by comparing experimental with simulated cyclic voltammograms obtained for solutions of [Os(NH3)(5)(OH2)](3+). Rate constants for the oxidation of [Os(NH3)(5)(OH2)](2+) by H+, CH3COOH (HA), and H2O were evaluated as k(H) = 17 M-1 s(-1), k(HA) = 1.2 M-1 s(-1), and k(H2O) = 4.1 x 10(-4) S-1. These values are shown to be compatible with the data in ref 1 and come close to accounting for the long-lived species described in ref 6 which may be [Os(NH3)(5)(OH2)](2+) itself. A revised value of 4.85 was measured for the pK(A) of [OS(NH3)(5)(OH2)(3+). Adsorption of an Os complex on the surface of mercury, but not glassy carbon, electrodes is shown to enhance greatly the rate with which protons undergo direct electroreduction.