HAMMETT reaction constants ρ for the basicities of several series of carbonyl compounds are known and have been found to correlate linearly with the infra-red frequencies of the carbonyl bond1. This constant is not available for monosubstituted benzophenones (that is, substituted in one ring only), because the basicities of only a few of them have been measured2,3 and the data obtained give a very poor Hammett plot. A knowledge of this constant would, however, be of interest in connexion with the linear correlation with infra-red frequencies. Therefore an attempt to estimate it indirectly is reported here.