Electrocatalysis for H+ reduction by a bis(2,2 ' :6 ' ,2 " -terpyridine)cobalt(II) complex ([Co(terpy)(2)(2+)])dispersed in a Nafion (R)) membrane coated on a basal-plane pyrolytic graphite (BPG) electrode was studied. It was found that the electrocatalysis for H+ reduction varies with the pH conditions employed. Under acidic pH conditions (pH less than or similar to 4), the electrocatalytic H+ reduction was induced by [Co(terpy)(2)(+)] (Co(I)), while under weakly acidic-neutral pH conditions (pH greater than or similar to 4), H+ reduction took place after forming [Co(terpy)(2)(+.)] Co(0)). The pH-dependent electrocatalysis was attributable to weak basicity of the electrogenerated Co(I): under low pH conditions (pH less than or similar to 4) Co(I) can coordinate H+ resulting in an intermediate (Co(I)-H). However, since the reduction potential of Co(II) is pH-independent above pH similar to 4, the H+ reduction took place by Co(0) via Co(0)-H. The electron transfer to generate catalytically active species at the polymer-coated electrode was also studied by potential-step chronoamperometry. Similar to the concentration dependence of the turnover number of the catalyst, the apparent diffusion coefficient (D-app) of the electrons decreased with increasing the complex concentration. It is most probable that the overall kinetics in the electrocatalytic H+ reduction is dominated by the reduction of the complex.