It is found that the Los Alamos (LANL) effective core potential scheme outperforms the Stevens effective core potentials (CEP-31G) for the calculation of gas-phase enthalpies of formation. Inspection of the results indicates that a major source of error is due to the use of effective core potentials for light main group elements (i.e., Z < 10) in the standard CEP-31G implementation. Hence, it is recommended that in modeling the thermochemistry of transition metal complexes that all-electron basis sets be used for the light main group elements when the most accurate energetics are required. (C) 2004 Elsevier B.V. All rights reserved.