Kinetic measurements of the acylation of toluene (2a) and p-xplene (2b), side-chain deuterated toluene (2a-d(3)), as well as perdeuterated toluene (2a-d(8)) and p-xylene (2b-d(10)) with the aroyl triflate 1 in 1,2-dichloroethane reveal a strong dependence of the isotope effect on reaction conditions. In the presence of trifluoromethanesulfonic acid (HOTf), the second-order rare constants k(H)/k(D) observed are in the order of 1.75-1.94, whereas in the presence of 2,4,6-tri-tert-butylpyridine (4) rate constants k(H)/k(D) Of 1.14-1.25 are found. The primary kinetic isotope effects observed correlate with the ortho/para ratio of the acylation of toluene. In the presence of 4 a relatively high percentage (similar to 30%) of ortho product is obtained, whereas under acidic conditions the ratio is only 10%. The correlation between isotope effects and isomer distributions is obviously due to the rate of deprotonation of the corresponding sigma -complex intermediates. Assuming a bent structure for sigma -complexes. the conformation giving deprotonation is preferred in the para sigma -complex in comparison with ortho complex.