We report on the performance of the third-order Douglas-Kroll ab initio model potential (DK3-AIMP) method-based electron-correlated spin-orbit calculations. Our treatment assumes that the problem can be separated into a spin-free correlation treatment and a spin-orbit calculation. The correlation effects were calculated using the multistate complete active space second-order perturbation method, and the spin-orbit effects were treated by means of the restricted active space state interaction spin-orbit method, where the spin-orbit effects were approximated by the Douglas-Kroll type of atomic mean-field spin-orbit method. We used our method for illustrative calculations on the ground and low-lying electronic states of thorium monoxide. For a proper description of the inner core region in the spin-orbit calculations, an auxiliary spin-orbit basis set was introduced. The DK3-AIMP-based electron-correlated spin-orbit calculations on ThO yield good agreement with corresponding all-electron results and with the available experimental data. This confirms that the DK3-AIMP method can be easily combined with highly accurate correlation treatments and relativistic effects, both of which are vital for studying the actinides. To our knowledge, the literature contains no references to AIMP calculations on the low-lying states of ThO. (C) 2003 American Institute of Physics.