Free ion formation in acetonitrile is examined through transient photoconductivity for a set of ketones excited at different wavelengths. According to the photophysical parameters of the ketones and the incident photon energy, two mechanisms can be operative: triplet-triplet annihilation (bimolecular process) and/or photoionization (monomolecular biphotonic process). By using a tunable laser, excited state mediated photoionization was studied. From the threshold energy (E-thr,) required for this process to occur, ionization potentials in solution (I-s) were deduced and compared to the corresponding values in gas phase (I-G). A simple energetic model enables the determination of the oxidation potential (E-ox) of the ketones that are compared to the corresponding values obtained through electrochemical measurements. (C) 2003 Elsevier B.V. All rights reserved.