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Thi. gas-phase pyrolysis of three secondary acetates in the temperature range of 290-350°C and the pressure range of 40-236 mm Hg reveals a homogeneous first-order law and unimolecular elimination. The rate constants are given by the following Arrhenius equations: isopropyl fluoroacetate: log kl (sec-l) = (12.83 =t 0.53) -(43,700 f 1400) cal/mo1/2.303RT isopropyl chloroacetate: log k , (sec-I) = (12.63 f 0.33) -(42,800 f 900) cal/mo1/2.303RT isopropyl iodoacetate: log k , (sec-l) = (13.09 f 0.23) -(43,300 j = 600) cal/mo1/2.303RTThe differences in rates I > Br > C1 > F apparently question previous assumptions where the pyi-olyses of esters follow the same sequence as acid strength. The experimental data give a good correlation with u* values, where p* = -1.85 a t 330°C. Some kind of discrete bond polarization a t the C , 4 bond of the acetate in the cyclic transition state seems to be involved in this type of reactions.