A complete active space valence bond (CASVB) method with orthogonal and non-orthogonal orbitals is applied to the collinear exchange reaction H + H2 → H2 + H and the unimolecular dissociation H2CO → H2 + CO. The bond nature during the reactions is analyzed using the occupation number (weight) of the valence bond resonance structures. The CASVB descriptions with orthogonal and non-orthogonal orbitals give a similar picture, although the ratio of covalent vs, ionic bonds are quite different. In the H2CO → H2 + CO reaction, the CH bond dissociation and HH bond formation occur after passing through the transition state. CASVB gives a clear understanding of reaction mechanisms in terms of competing bonding schemes whose weights change along the intrinsic reaction coordinate.