High-level ab initio calculations are performed on HgCH3 and HgCH3+. For HgCH3+, the obtained geometry and vibrational frequencies could be compared to Raman studies of HgCH3+ interacting with biomolecules. HgCH3 was found to be a weakly bound species with a binding energy, D-0, of only 3 kcal mol(-1). The first adiabatic ionization energy was calculated to be 7.3 +/- 0.1 eV, corresponding to an (a(1))(-1) ionization. The heats of formation were deduced for both the neutral and cation at 298 K, giving values of DeltaH(f) = 45 +/- 2 and 235 +/-2 kcal mol(-1), respectively. (C) 2003 Elsevier B.V. All rights reserved.