The electronic structure and decay dynamics of a long-lived excited state in CdS quantum dots (Q-CdS) were investigated in a novel way by a time-resolved optoelectrical method. Q-CdS was adsorbed as a monolayer on a gold electrode and used in a photoelectrochemical cell, the gold/Q-CdS/electrolyte structure forming a double-barrier tunnel junction. The rates of photoinduced tunneling processes were measured by intensity-modulated photocurrent spectroscopy (IMPS). These tunneling processes compete with intraparticle decay of a long-lived excited state. In this way the excited-state dynamics could be studied. From a scan of the gold Fermi level, it was deduced that the long-lived excited state consists of an electron in the LUMO and a hole trapped in a level high in the bandgap.