The effect of buffer pK(a) on the mechanism of electrochemical hydrogen evolution catalyzed by a cobalt porphyrin peptide (CoMP11-Ac) at constant pH is presented. The addition of buffer to CoMP11-Ac in water and KCl leads to an enhancement of the catalytic current of up to 200-fold relative to its value in the absence of a buffer. Two distinct catalytic regimes are identified as a function of the buffer pK(a). In the presence of buffers with pK(a) <= 7.4, a fast catalysis regime limited by diffusion of buffer is reached. The catalytic half-wave potential (E-h) shifts anodically (from -1.42 to -1.26 V vs Ag/AgCl/KCI(1M))( )as the buffer pK(a) decreases from 7.4 to 5.6, proposed to result from fast Co(III)-H formation following the catalysis-initiating Co(II/I) reduction. With higher-pK(a) buffers (pK(a) > 7.7), an E-h = -1.42 V, proposed to reflect the Co(II/I) couple, is maintained independent of the buffer pk, consistent with rate-limiting Co(III)-H formation under these conditions. We conclude that the buffer species pK(a) impacts catalytic current and potential and the rate-determining step of the reaction.