The effective electrode area and charge density of iridium metal and anodically activated iridium has been measured by optical and electrochemical techniques. The degree of electrode activation could be assessed by changes in electrode colour. The reduction charge, activation charge, number of activation pulses and charge density were all strongly correlated. Activated irididrn showed slow electron transfer kinetics for reduction of a dissolved redox species. At fast voltammetric scan rates the linear diffusion electroactive area was unaffected by iridium activation. At slow voltamrnetric scan rates, the steady state diffusion electroactive area was reduced by iridium activation. The steady state current was consistent with a ring electrode geometry, with lateral resistance reducing the eleCtrode area. Slow electron transfer on activated iridium would require a larger overpotential to reduce or oxidise dissolved species in tissue, limiting the electrodes charge capacity but also reducing the likelihood of generating toxic species in vivo. (C)2017 Elsevier Ltd. All rights reserved.