Graphene oxide (GO) nanosheets have received a great deal of attention for a wide range of applications from optoelectronic devices to biological sensors. We now report a mechanistic study of the interfacial electron transfer (ET) processes between organic dye molecule, 9-phenyl-2,3,7-trihydroxy-6-fluorone (PF), and nanometre-sized GO sheets using ensemble-averaged and single-molecule spectroscopies. The ET dynamics was characterized by the direct observation of the PF radical cation during the laser flash photolysis, and its reaction rate was determined to be ~10(11) s(-1). The single-molecule fluorescence spectroscopy was utilized to clarify the heterogeneous nature of the interfacial ET within individual composites. Their fluorescence lifetimes and spectra were found to vary from composite to composite, possibly due to the different local structures and molecular interactions. The autocorrelation analysis of fluorescence intensity trajectories also revealed the temporal fluctuation of the ET reactivity.