We present electrochemical analyses of the catalysis of dioxygen reduction by Thermus thermophilus strain HB27 laccase on ketjen black substrates. Our cathodes reliably produce 0.56 mA cm(-2) at 0.0 V vs AglAgCl reference at 30 degrees C in air-saturated buffer, under conditions of nonlimiting O-2 flux. We report the electrochemical activity of this laccase as a function of temperature, pH, time, and the efficiency of its conversion of dioxygen to water. We have measured the surface concentration of electrochemically active species, permitting the extraction of electron transfer rates at the enzyme-electrode interface: 1 s(-1) for this process at zero driving force at 30 degrees C and a limiting rate of 23 s(-1) at 240 mV overpotential at 50 degrees C.