The hydrolysis of tetravalent plutonium in 0.5 M HCl/NaCl has been investigated by a combination of spectroscopy and redox potential measurements in the concentration range from 10(-5) to 4 x 10(-4)M and at pH(c) 0.3-2.1. Redox potentials were both measured and calculated from the plutonium oxidation state distributions using well-known equilibrium constants for the redox reactions of plutonium. The plutonium oxidation states in solution were quantified by spectroscopy. The p, values calculated from the plutonium redox couples Pu(V)/Pu(VI) and PuO2(am,hyd)/Pu(V) were found to be consistent with the measured p(e) values whereas those calculated from the Pu(III)/Pu(IV) couple were found to deviate, suggesting that the formation constants selected in recent reviews for the mononuclear hydroxide complexes of Pu(IV) are overestimated. Combining the redox potentials and the spectroscopically determined Pu(III) and Pu(IV) concentrations, the formation constants of the first, second, and third mononuclear hydroxide complexes Pu(OH)(n)(4-n) at I= 0.5 M and 23 +/- 2 degrees C are calculated and corrected to zero ionic strength using the specific ion interaction theory (SIT): log beta degrees(11) = 14.0 +/- 0.2, log beta degrees(12) =26.8 +/- 0.6, and log degrees(13) = 38.9 +/- 0.9.