Three complete basis set models of Petersson cr nl. [J. Chem. Phys. 104, 2598 (1996)], CBS-QI CBS-q, and CBS-4, have been assessed on the G2 neutral test set of 148 molecules [J. Chem. Phys, 106, 1063 (1997)]. The average absolute deviations with experiment of the calculated enthalpies of formation from the three CBS methods are 1.57 kcal/mol (CBS-Q), 2.13 kcal/mol (CBS-q), and 3.06 kcal/mol (CBS-4). The maximum deviations of the methods are 11.2, 10.3, and 14.4 kcal/mol. respectively. The most accurate method, CBS-Q, has an average absolute deviation similar to that of G2 theory. The three CBS methods have also been assessed on a 40 molecule set using isodesmic bond separation reactions to calculate enthalpies of formation. There is a significant improvement in the accuracy of the enthalpies compared to these calculated using atomization energies? although not as much as for G2 theory. Ln a test on naphthalene, enthalpies calculated using the CBS methods have large deviations. The CBS-Q method has a deviation of 28.7 kcal/mol and, surprisingly, the deviation increases to 34.3 kcal/mol when isodesmic bond separation reaction energies are used.