To develop the capability to create diamond electrodes with greater functionality, covalent modification was carried out on homoepitaxial single-crystal diamond electrode surfaces. (100) and (111) single-crystal boron-doped diamond electrodes were first prepared homoepitaxially, then subjected to oxidative treatments, and the functional groups on the oxidized surfaces were analyzed by employing X-ray photoelectron spectroscopy (XPS). Based on the results, we conclude that both singly and doubly bonded oxygen groups (C-O and C=O) were generated on the anodically treated (100) diamond electrode surfaces, whereas only singly bonded oxygen groups (C-O) were generated on the (111) surfaces. Second, selective surface modifications of anodically treated (100) and (111) diamond surfaces with 2,4-dinitrophenylhydrazine and 3-aminopropyltriethoxysilane moieties were carried out. Based on the tendencies of these surface modifications, together with the XPS results, we propose that carbonyl and hydroxyl groups are generated on anodically treated (100) diamond surfaces, and hydroxyl groups are mainly generated on anodically treated (111) diamond surfaces. The study of chemical modification of single-crystal diamond electrode surfaces should be useful, not only for creating new types of functional electrodes, but also for better understanding the properties of polycrystalline diamond electrodes and their possible applications. (C) 2004 The Electrochemical Society.