The underpotential deposition (UPD) of thallium onto a polycrystalline gold macroelectrode or gold nanoparticles (10 ± 5 nm in diameter) supported on multiwalled carbon nanotubes (MWCNTs) is compared both in the presence and the absence of adsorbed chloride anions. On the polycrystalline gold macrodisc electrode the UPD of thallium is observed. However, on the 10 nm gold nanoparticles supported on MWCNTs no UPD of thallium is observed despite several control experiments which confirmed that the gold nanoparticles were indeed electroactive. This is compared to the reported behaviour of larger 30–60 nm diameter gold nanoparticles at which UPD of thallium is also found to occur and the possible reasons for this are discussed. This report provides an example where the properties of metal nanoparticles differ markedly from the bulk metal and the properties of the nanoparticles vary significantly with the size of the nanoparticles.