Two examples of selective electrochemical modification of individual nanoscale objects are presented, namely the electroless metallization of a plant virus, and the coupling of substituted phenyl residues to single-wall carbon nanotubes (SWCNTs). The electroless deposition of metal was achieved selectively on the inner or outer surface of tobacco mosaic virus (TMV) particles. Covalent modification of SWCNTs, deposited on a Si/SiO2 substrate, was performed successfully via reductive coupling of a 4-nitrobenzenediazonium salt in non-aqueous medium. An organic layer with a thickness of up to several nanometers could be deposited on isolated tubes and thin bundles.