We have calculated the vertical excitation energies and oscillator strengths of the lowest energy electronic transitions in sulfuric acid (H2SO4) and sulfur trioxide (SO3) with a range of ab initio methods. We have found that the highest level calculations with the complete active space self-consistent field and multireference configuration interaction (MRCI) methods predict transition energies much lower than those previously calculated with the simpler configuration interaction-singles method. The MRCI calculated electronic transitions for SO3 are in good agreement with the experimental results, whereas electronic transitions in vapor phase H2SO4 have yet to be observed. Our MRCI results suggest that the lowest lying electronic excitation in H2SO4 occurs around 144 nm and that the cross section in the actinic region is very small. (C) 2003 American Institute of Physics.