The reversible formal potentials of auracyanin A and auracyanin B, two closely related "blue" copper proteins from the photosynthetic bacterium Chloroflexus aurantiacus, have been determined by protein film voltammetry in the range 4<or=pH<or=9. At pH 7 in 0.1 M NaCl, the values of for auracyanin A and auracyanin B are 205+/-7 mV and 215+/-7 mV, respectively, versus the standard hydrogen electrode. In both cases there is a smooth but non-sigmoidal change in from approximately 190 mV at pH 9 to approximately 240 mV at pH 4. The small changes in as a function of pH indicate that auracyanin A and auracyanin B differ from those "blue" copper proteins in which the Cu site in the reduced (Cu(I)) state switches to a redox-inhibited form at low pH. For auracyanin A, the results obtained by protein film voltammetry are closely similar to those obtained by the conventional spectroelectrochemical method. The findings are discussed in relation to the putative role of auracyanin in biological electron transfer.