Résumé : Rate constants and Arrhenius parameters pertaining to the bimolecular exchange between 1‐alkyl‐4‐bromo‐3,5‐dinitro‐benzenes and potassium iodide, in dry acetone, have been determined. The alkyl radical is sucessively: H, CH3, (CH3)3C. The experimental data give a measure of the polar effect of these alkyl groups located para to the seat of substitution. Copyright © 1955 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim