AbstractA set of 4‐monosubstituted cyclopentenes, , were synthesized and their relative rates (kX/kH) for bromination and chlorination were determined in methanol, ethanol and acetic acid at 25 °C by competitive method. log(kX/kH) for most of the substituents can be correlated by means of Taft's equation, log(kX/kH) = ρIσI+C. In methanol ρ I, Br 2= −2·91, ρ I, Cl 2= −0·49, in ethanol ρ I, Br 2= −3·07, ρ I, Cl 2= −0·70 and in acetic acid ρ I, Br 2= −1·64, ρ I, Cl 2= −0·65. The presence of C(<0) is due to a constant steric effect. The deviation of X = H is ascribed to the absence of the steric effect and that of X = CO2Me and CO2Et is accounted for in terms of anchimeric assistance. Fo