Visible light irradiation of an aqueous solution of a platinum ion and a non-ionic surfactant, poly(ethylene glycol) monolaurate, gives a colloidal dispersion of nanoscopic platinum clusters protected by the surfactants. The structure of the surfactants protecting the platinum clusters is investigated by a nuclear magnetic resonance (NMR) technique. In the C-13 NMR spectra, all carbon signals of the hydrophobic alkyl chain of the surfactant decrease in height with increasing concentration of the platinum cluster. This indicates that the platinum cluster particle is not incorporated into the central hydrophobic hole of the surfactant micelle, but is protected by the adsorption of surfactant molecules on the surface of the cluster particle at the hydrophobic part.