Calculations on Rydberg states are performed using quantum Monte Carlo (QMC) methods. Excitation energies and singlet-triplet splittings are calculated for two model systems, the carbon atom (3P and 1P) and carbon monoxide (1Σ and 3Σ). Kohn-Sham wave functions constructed from open-shell localized Hartree-Fock (OSLHF) orbitals are used as trial and guide functions. The fixed-node diffusion quantum Monte Carlo (FN-DMC) method depends strongly on the wave function’s nodal hypersurface. Nodal artefacts are investigated for the ground state of the carbon atom. Their effect on the FN-DMC results can be analyzed quantitatively. FN-DMC leads to accurate excitation energies but to less accurate singlet-triplet splittings. Variational Monte Carlo (VMC) calculations are able to reproduce the experimental results for both the excitation energies and the singlet-triplet splittings.