Doubling the ethyl acetate concentration doubles the speed of reaction, and doubling the hydrogen ion concentration does the same thing. Lowering the pH by one unit (a tenfold increase in [H⁺]) makes the hydrolysis ten times as fast.

The rate therefore is sensitive to the concentration of a substance that does not appear in the overall reaction and is not represented in the equilibrum-constant expression:

This behavior is a clue that catalysis is involved. It occurs because the rate of the reverse reaction, synthesis of ethyl acetate from ethanol and acetic acid, also is proportional to hydrogen ion concentration:

Protons, or hydrogen ions, catalyze both the forward and reverse reactions to the same degree, and proton concentrations cancel from the overall equilibrium expression.