CASSCF and QCISD calculations were carried out on the (X) over tilde A(i) stale of Cl2O and the (X) over tilde(2)B(1), (A) over tilde(2)B(2), (B) over tilde(2)A(1), and (C) over tilde A(2) states of Cl2O+ in order to obtain their minimum-energy geometries and harmonic vibrational frequencies. Cl2O+ <-- Cl2O (X) over tilde(1)A(i) vertical and adiabatic ionization energies were also computed at the RCCSD(T)//QCISD level. Spectral simulatians based on thr ab initio results were performed for the first four bnnds of the He I photoelectron spectra of Cl2O, Iterative Franck-Condon analyses were also carried out. Comparison between the simulated and the observed spectra gave, for the first time, experimentally derived geometries, for the (X) over tilde(2)B(1), and (A) over tilde(2)B(2), and (C) over tilde(2)A(2) cationic states. The adiabatic ionization energy position and the vibrational assignments of the Cl2O+ (A) over tilde(2)B(2) <-- Cl2O (X) over tilde(1)A(i) band were revised, based on spectral simulations. which included "hot" bands.