AbstractThe bonding nature of the canonical molecular orbitals 2σg, 2σu and 3σg in the molecules N2,O2, F2and the related analogous molecular orbitals in the molecules P2and CO, is analysed using Weinhold's natural bond orbital set. When the canonical molecular orbitals can be well localized into natural bond orbitals, the covalent bond can be completely attributed to the bonding type natural bond orbitals. The decomposition of canonical molecular orbitals into the natural bond orbital basis then gives the weighted bond order as the component of the bonding portion in the canonical molecular orbital. The weighted bond order results match the photoelectron spectroscopy assignment quite satisfactori