AbstractThe apparent acidity constants of substituted β‐phenylthio‐, β‐phenylsulfinyl‐and β‐phenylsulfonyl‐acrylic acids (cisandtrans) with substituentsp‐OCH3,p‐CH3, H,p‐Cl,m‐NO2andp‐NO2have been determined in 50% (by vol.) aqueous ethanol. The acidity constants in the six series obey theHammettequation with satisfactory to excellent fit. The values of the reaction constants δ indicate that substituent effects are better transmitted by S than by SO and SO2, and they do not differ substantially forcisandtransseries.The β‐arylthioacrylic acids are more acidic intrans‐ than incis‐configuration, while the opposite holds for the β‐arylsulfinyl‐ and β‐arylsulfonyl‐acrylic acids. These differences are discussed in terms of steric inhibition of resonance and electrostatic effects through space.Comparison between the acidity constants oftrans‐β‐substituted acrylic acids with those ofpara‐substituted benzoic acids reveals that a carbon‐carbon double bond is 1.9 times as effective as the ph