Cesium and cesium oxide clusters have been ionized in a one-photon process by means of a tunable cw dye laser and detected in a mass spectrometer. Mass spectra obtained using different photon energies and photoionization spectra of 7 metal and 86 oxidized metal clusters (in a mass range from 1 to 2024 amu and a maximum energy of 3.1 eV) are presented. Vertical ionization potentials were determined from the spectra. The interpretation of the ionization threshold is given in terms of the displaced harmonic oscillator model which leads to a relation between adiabatic and vertical ionization potentials (IP). The ionization energies of cesium clusters Csn decrease with cluster size. An unusually low IP was observed for the enneamer Cs9 and for the cesium monoxide Cs11O. With increasing oxidation of the cesium metal clusters the IP decreases (suboxides) reaches a minimum at Cs(Cs2O)n and then increases (superoxides).