MR-CISD and MR-CISD + Q calculations have been performed for the vertical excitations of protonated formaldehyde in comparison to formaldehyde. Singlet and triplet states have been investigated. It is shown that the protonation causes the Rydberg states to be shifted to higher energies by several eV. This finding is discussed by means of the Rydberg formula in terms of quantum defects for the two lowest vertical ionization energies. For protonated formaldehyde the pi-pi(*) valence state is energetically the second lowest state at 9.80 eV, about 1.50 eV below the first Rydberg n-3s state. This finding is in strong contrast to the case of formaldehyde where the pi-pi(*) state is embedded within a series of Rydberg states. (C) 2003 Elsevier Science B.V. All rights reserved.