This dissertation presents an experimental photoionization study of the cerium isonuclear sequence ions in the energy range of the 4d inner-shell giant resonance. In addition, single and double photoionization and photofragmentation cross sections of the cerium endohedral ion Ce@[special characters omitted] were also measured and studied in the 4d excitation-ionization energy range of cerium. Relative and absolute cross-section measurements were performed at undulator beamline 10.0.1 of the Advanced Light Source (ALS) for nine parent cerium ions: Ce⁺ − Ce⁹⁺. Double-to-single ionization cross-section ratios were measured for photoionization of the endohedral Ce@[special characters omitted] and empty fullerene C[special characters omitted] molecular ions. The merged ion and photon beams technique was used to conduct the experiments. Multiconfiguration Hartree-Fock calculations were performed as an aid to interpret the experimental data. Four Rydberg series for 4d → nf (n ≥ 4) and 4d → np (n ≥ 6) autoionizing excitations were assigned using the quantum defect theory for the Ce³⁺ photoionization cross section. The experimental data show the collapse of the nf wavefunctions (n ≥ 4) with increasing ionization stage as outer-shell electrons are stripped from the parent ion. The nf orbital collapse occurs partially for Ce²⁺ and Ce³⁺ ion and completely for Ce⁴⁺, where these wavefunctions penetrate the core region of the ion. A strong contribution to the total oscillator strength was observed in the double and triple photoionization channels for low charge states (Ce ⁺, Ce²⁺, and Ce³⁺), whereas most of the 4d excitations of the higher charge states decay by ejection of one electron.