Photoelectron spectra of SO3- were recorded at 266 and 355 nm to study photodetachment of the SO3- anion ((2)A(1)) to the ground state of neutral SO3 ((1)A'(1)). A long vibrational progression in the 355 nm spectrum is attributed to excitation of the umbrella mode, nu (2), consistent with predictions that C-3 nu symmetry SO3- yields D-3h symmetry SO3 upon photodetachment. At 266 nm, photodissociation of SO3- to SO2 + O- was also observed. The geometry and normal-mode frequencies of SO3- and SO3 as well as the adiabatic electron affinity (AEA) and vertical detachment energy (VDE) of SO3 have also been calculated with ab initio (MP2 and CCSD(T)) and DFT methods. Using theoretical predictions and experimental data, Franck-Condon simulations of the photoelectron spectra were found to be in good agreement with experiment. The calculated AEA agreed well with experiment, but the VDE was found to be less accurate, presumably because of the large geometry change between anion and neutral.