The adsorption of benzene and cyclohexene was studied at the stepped Pt(332) = Pt(s)[6(111) x (111)] and at the Pt(111) electrode surface using differential electrochemical mass spectrometry (DEMS). The experiments involve desorption and hydrogenation in the hydrogen region, oxidation in the oxygen adsorption region and non-reactive displacement by underpotential deposition of Cu. The results are compared with those obtained previously at Pt(111) and at Pt(110). The total coverage is similar on all surfaces and for both molecules (0.3-0.4 nmol cm(-2)). Benzene adsorbed at the Pt(332) electrode desorbs nearly quantitatively as cyclohexane during potential sweeps to the hydrogen region below 0.05 V, similar to the case of the Pt(110), whereas benzene is desorbed without hydrogenation from Pt(111) below 0.2 V. Therefore, the step density of the Pt(332) is sufficiently large or the ratio of the terrace width to the size of the molecule is small enough so that a larger number of the molecules are influenced by the steps and are hydrogenated. Decoration of the steps by Cu previous to adsorption of benzene or cyclohexene reduces the adsorbate coverage by a factor of 2, although only around 20% of all adsorption sites are blocked by Cu. This shows, that the rows of Cu are a hindrance for a dense adlayer of the adsorbate. Hydrogenation of the adsorbate is largely diminished, proving that the step sites are the catalytic sites for the hydrogenation reaction. Whereas previous experiments on the displacement of the adsorbate on Pt(110) by UPD of Cu had shown that cyclohexene is dehydrogenated to benzene upon adsorption, similar experiments on Pt(111) show that less than one third of the total adsorbate (0.08 nmol cm(-2)) are displaced as cyclohexene. Only one tenth of the adsorption product of cyclohexene on Pt(332) is displaceable, solely as benzene, under these non-reactive conditions. Therefore, dehydrogenation is also only occurring at the step sites, but adsorption leading to another, more abundant adsorption product, which is not displaceable, seems to be faster. (C) 2003 Elsevier Ltd. All rights reserved.