Plasma-treated cobalt metalloporphyrins have recently been proposed as electrocatalysts for the oxygen and oxygen peroxide reduction reaction. Whereas the effects of plasma treatment on the elemental composition of the surface of catalysts have been investigated, the effects of plasma treatment on the morphology of catalysts have not yet been studied. In this study, plasma modified nanosized structures of cobalt tetramethoxyphenylporphyrin (CoTMPP) molecules arising from the deposition of a porphyrin solution on an a-C:H film are investigated using an atomic force microscope (AFM). Additionally, the effects of plasma treatment on the structure of porphyrin molecules are studied by using ultra violet visible (UV-vis) absorption analysis. The investigations reveal the morphological changes which accompany the transformation of CoTMPP into the final catalytic material. First, the large CoTMPP aggregates are split into smaller ones. Second, the CoTMPP layer appears to be sublimated after plasma treatment. Sublimated CoTMPP molecules can be decomposed by plasma and after redeposition can form catalytic active fragments. (C) 2010 Elsevier B. V. All rights reserved.