Gaussian-2 (G2) theory has been extended to molecules containing the third-row nontransition elements Ga-Kr. Basis sets compatible with those used in G2 theory for molecules containing first- and second-row atoms have been derived. Spin-orbit corrections for atoms and molecules having spatially degenerate states (P-2, P-3 for atoms and (2) Pi for molecules in this work) are included explicitly in the G2 energies. The average absolute deviation from experiment for 40 test reactions is 1.37 kcal/mol. In contrast to the situation for first- and second-row molecules, inclusion of spin-orbit effects is very important in attaining accurate energies for molecules containing third-row atoms. Without spin-orbit effects, the average absolute deviation is 2.36 kcal/mol.