The alkaline spectroscopic transition of the copper at the active site of Cu,Zn superoxide dismutase has been reexamined by room temperature EPR, in order to correlate it with the inhibition of the enzyme activity at high pH. The EPR transition is governed by a single prototropic equilibrium, with pK values of 11.3 and 11.1 for ox and shark superoxide dismutase, respectively. This result suggests possible contributions of changes of the copper environment to the higher pK of the activity/pH curve. When Arg141 was chemically modified by phenylglyoxal treatment of the ox protein, a lower pK value (10.8) was obtained, indicating that Arg141 is involved in the observed modifications of the EPR spectra.