[en] Chlorination of toluene by tert-butyl hypochlorite involves the attack of t-BuO on the benzylic hydrogen; isotope effects can be measured by comparing the activity of the product t-BuOH with that of PhCH/sub 3-t/. At 40⁰C, k/sub H//k/sub T/ = 11.5; substituted toluenes give similarly rather large values of k/sub H//k/sub T/, which increase slightly with sigma. In contrast, Cl attack gives k/sub H//k/sub T/ = 3.7, in rough agreement with earlier deuterium effects. Literature values for the isotope effect in the nearly equally exothermic attack by CH₃ and the more exothermic attack by C₆H₅ are closer to the t-BuO value than to the Cl value. Earlier large values of isotope effect in bromination are confirmed with tritium. Secondary effects of tritium in the t-BuO attack were measured and are undetectable. It is postulated that the symmetry effect on the isotope effect is important only when one of the elements flanking hydrogen in the transition state is in the second row or beyond