AbstractThe apparent acidity constants of a series ofortho‐, α‐ and/or β‐substituted cinnamic acids (cis‐ andtrans‐), with substituents CH3, Br and Cl, have been measured in 50% (by vol.) aqueous ethanol at 25°.The data on α‐ and/or β‐substitution by Br and Cl are discussed on the basis of electronic substituent effects. Similar substitution by CH3invokes the enhanced secondary steric effect of this group and a large departure from additivity of substituent effects is observed in α,β‐dimethyl‐trans‐cinnamic acid.The acidity constants of β‐substitutedcis‐cinnamic acids follow theHammettequation with satisfactory fit.Ortho‐substitution by Cl incis‐cinnamic acid probably implies a direct field effect betwe