Substituted 2-aryl-propionaldehydes tautomerize in dimethylsulfoxide to the corresponding enoles. The ratio of the tautomers can be described by Hammett σ-values, as in the case of imin-enamine-tautomeric 2-aryl-propionaldehyde methylimines. This is the first example of a linear free energy relationship between the two forms of tautomerism.