Systematic investigations of the kinetics of the reactions of ethyl bromoacetate (EBA) and of picryl bromide (PB) with 2-phenylthiazolidines (PT) in 100% ethanol were undertaken at 30, 35, 40, and 45 °C. The most incisive information concerning the Hammett equation comes from the extensive studies on the substituted 2-phenylthiazolidines, establishing linear Hammett plots with negative ρ values. The kinetic data, plotted in a Brønsted fashion against pKaof the corresponding thiazolidinium ions, yield linear relationships. The activation parameters were computed. The linear relationships between ΔH≠and ΔS≠signify a consistent mechanism throughout the series.