Infrared spectra of three new thorium oxide species have been obtained in argon and neon matrixes. All of the products are experimentally characterized using isotopic oxygen samples with the aid of electronic structure calculations. Ground state thorium atoms react with O-2 to form the ThO2 molecules, which can dimerize to give Th2O4 products. Th2O4 is predicted to have nonplanar C-2h, symmetry for its closed shell singlet ground state. The rhombus-shaped Th2O2 molecule in the (1)A(g), (D-2h) ground state is also observed and its formation is proposed via the reaction of Th-2 with O-2. In addition, electron capture of neutral thorium dioxide results in the formation of the ThO2- anion. It is predicted to have a doublet ground state with a geometry similar to that of the neutral ThO2 molecule. Electronic structure calculations on the unobserved Th2O and Th2O3 molecules are also provided.