The WIMPER (2) approximation to the logarithm of the reduced partition function ratio is used to develop analytical expressions for the contribution of individual molecular vibrational force constants to the logarithm of the reduced partition function ratio of isotopic molecules. The method inherently accounts properly for contributions from quadratic terms of the form βpqxpxq, which are counted twice in any numerical method. The zero order (diagonal F–diagonal G) approximation leads to values of ln(s/s′)f0 which are always larger than the exact value when the site of isotopic substitution is a central atom. In the latter case the first order correction (j=1 term) is always negative because of the sign of the off- diagonal G matrix element. As a result of the convergence properties of the WINIMAX polynomials the sign of the j=1 correction to ln(s/s′)f0 usually determines the final sign of the correction to ln(s/s′)f0 associated with any off-diagonal F matrix element. Calculations are given for the contribution of each force constant to ln(s/s′)f, ln(s/s′)f0, and the j=1 and j=2 correction terms for D/H, 13C/12C, and 18O/16O isotopic substitutions. Molecules studied include H2O, CO2, H2CO, CH4, C2H4, C2H6, and C6H6. The contributions for off-diagonal F matrix elements for D/H substitution are found to be small in agreement with previous work. The largest effects associated with off-diagonal F matrix elements arise in the torsion of ring structures. These are illustrated in detail by numerical evaluation of each of the terms contributing to ln(s/s′)f for 13C/12C for the out-of-plane vibrations in benzene. Even in the latter case the WIMPER (2) method leads to approximate values of ln(s/s′)f within 3.6% of the exact value. When the j=2 correction is significant, it is usually associated with the coupling of internal coordinates through quadratic cross terms in the kinetic energy (g2ij fii fjj) or the coupling of an internal coordinate (gii fii) with a cross term in either the kinetic energy (gij) or potential energy (fij). It is shown by numerical calculation and through an analysis of the ground state contribution to ln(s/s′)f0 that both carbon and deuterium isotope effects at 300 °K associated with C–H stretching motions and H–C–H bending motions can be represented as simple functions of the square roots of the respective force constants. The proportionality constants are calculated a priori in good agreement with detailed calculations from a series of molecules with C–H bonds. The latter affords a simple method of calculation of lnc(u0) values directly from C–H stretching and H–C–H bending force constants.