Using electronic absorption spectroscopy, and steady-state and time-resolved fluorescence, we studied the singlet state properties of dendro[60]fullerene, a C-60 methanoderivative whose addend is a water-soluble dendrimer. Fluorescence quenching, especially by halide ions in water, was characterized at several temperatures. From a new relation for the activation energy of a diffusion-influenced reaction and from a comparison with the quenching in organic solvents, it is concluded that the quenching occurs by the external heavy-atom effect. An efficient photoinduced oxidation of iodide in aerated solutions and in the presence of a fullerene is also reported., The photoinduced oxidation mechanism exclusively involves ground-state oxygen.