This hypothesis states that, if in an acid catalysed reaction, \(\log _{10}k_{1}\) (first-order rate constant of the reaction) is linear in \(H_{0}\) (Hammett acidity function), water is not involved in the transition state of the rate-controlling step. However, if \(\log _{10}k_{1}\) is linear in \(\log _{10}\left[\rm{H}^{+}\right]\) then water is involved. This has been shown to be incorrect by Hammett himself.