Perturbation theory for nonsymmetric matrices is discussed for the GF matrix for molecular vibrations. As a simple extension of early results, two approaches are given: one a direct diagonalization of the nonsymmetric matrix, the other a presymmetrization. Presymmetrization of the GF matrix, well known for DeltaF, is also described here for DeltaG. It permits the use of standard perturbation theory for symmetric matrices. Application of the second-order expression for the DeltaG case to the determination of the frequencies of many ozone isotopomers is given elsewhere.