It is shown that the average CH stretching frequencies in the fluoroaromatic series C 6H 6−nF n depend on the fluorine substitution patterns, being given by v̄ = v̄ B + n o X o + n m X m + n p X p. Two variants on the general quadratic force field, which are taken to be transferable between members of the series, have been fitted to the experimental data. Each ortho fluorine substituent increases the diagonal CH stretching force constant by 1.0%. The dependence on meta and para substituents is not as obvious as with the average stretching frequency due to the rather large variance. A comparison with existing correlations between average stretching frequencies and bond length data indicates that an ortho fluorine atom leads to a CH bond shortening of 0.0017 A. Such information could prove of value in microwave structural investigations.