IrO2 electrodes used as oxygen evolving anodes were studied by electrochemical impedance spectroscopy (EIS) to establish criteria for the assessment of their electroactive surface area and their intrinsic catalytic activity. Two types of IrO2-coated electrodes were used: anodically grown oxides on an It substrate and commercially available IrO2 nanoparticles, dispersed in a Nafion (R) polymer electrolyte layer on a glassy carbon substrate. EIS data indicate that both formulations behave as porous electrodes and their response can be described by variants of the classic transmission line model both at potentials outside and within the oxygen evolution reaction (OER) potential range; however, in the latter case, useful parameters could also be obtained by the use of common Randles models. An alternative approach to obtain capacitance values (free from the assumptions and geometric formulations of a model) has been based on plots of -1/(omega Z(Im)) vs. Z(re) that acquire or approach a limiting total capacitance value C-t at low or intermediate frequencies; the latter scales well with the charge associated with IrO2 surface electrochemistry and with oxide layer thickness or oxide particle loading. C-t can be taken as a measure of the catalyst electroactive surface area and the product RctCt (where R-ct is the OER charge transfer/polarization resistance) as the appropriate parameter to characterise the catalyst intrinsic catalytic activity. C-t can also be used to correct direct current vs. electrode potential OER data for electroactive surface area effects. (C) 2015 Elsevier B.V. All rights reserved.