A radical trapping technique employing 1,1,3,3-tetramethylisoindolin-2- yloxyl (1) as scavenger has been used to examine the regioselectivity for tail or head addition (kT/kH) by t-butoxy radicals to chloroethylene (5), fluoroethylene (6), 1,1-difluoroethylene (7) and 1,1,2- trifluoroethylene (8) at 30-75°C. Addition rates relative to that for ethylene (4) have also been evaluated. An atypical preference for head addition was observed for olefins (7) and (8). In the case of (7) the regioselectivity was shown to vary with temperature according to ���������������������� In(kT/kH) = (1.06�0.01)-(800�40)/TA comparison of the selectivity with that of other radicals indicates that t-butoxyl exhibits nucleophilic character in its addition to halo-olefins.