The B3LYP functional was re-optimized using partially composite methods for the calculation of standard enthalpies of formation. Two initial methods (B3LYP-MCM1 and B3LYP-MCM2) differing in the type and number of optimized parameters were analyzed using B3LYP/6–31 + G(2df,p) as the reference energy. For the first method (B3LYP-MCM1), the exchange-correlation and higher-level correction parameters (HLC) were optimized and for the second method (B3LYP-MCM2), in addition to the previous parameters, scaling of the basis functions responsible for large errors in the enthalpy of formation were also optimized. The best parameters were also used as adapted functionals generating two other methods referred to as: B3LYP-MF1 and B3LYP-MF2. The best-performing results (B3LYP-MCM2 and B3LYP-MF2) presented mean absolute errors near 2.3 kcal mol−1 for the G3/05 test set. This is a significant improvement when compared with the respective results from B3LYP/6–31 + G(2df,p), which yielded a mean absolute error of 5.3 kcal mol−1. The errors were larger for B3LYP-MCM1 (4.17 kcal mol−1) and B3LYP-MF1 (3.98 kcal mol−1). The scaling of the experimental atomization energies used for the calculation of enthalpy of formation was also tested for all four methods. This empirical adjustment reduced the errors to 2 kcal mol−1. The uncertainty of the best results with 95% confidence tended to be ± 5.5 kcal mol−1. Substantial improvements were associated with the basis set adjustments.