Photostimulated additions of tert-butylmercury halides to 1,6-dienes and enynes, including (CH2=HCH2)(2)X with X = CH2, C(COEt)2, O, or NCH2CH=CH2, form the primary alkylmercury halides resulting from 5-exo-cyclizations of the adduct radicals, t-BuCH(2)CH . CH(2)XCH(2)CH=CH2. The kinetic chain lengths of these free-radical processes are low and in the range of 3-7 at 35 degrees C in Me(2)SO. 5-Exo-cyclizations with kinetic chain lengths of similar to 100 are observed in the photostimulated addition of t-BuHgI to CH2=CHP(O)(OR)OCH2CH=CH2 (R = H or allyl), CH2=CHC(O)N(R)CH2CH=CH2 (R = Me, Ph, or CH2CH=CH2), or CH2=CHC(O)OC(Me)(2)CH=CH2. The radicals t-BuCH(2)CH . C(O)C(Me)2(CH2)(n)CH=CH2 cyclize mainly in the 6-endo mode for n = 1 and more rapidly in the 6-exo mode for n = 2, while the radicals t-BuCH(2)CH . C(O)OC(Me)2(CH2)(n)CH=CH2 cyclize only in the exo mode for n = 0 or 1 but with a faster rate for 5-exo-cyclization. Gem-dimethyl substitution adjacent to the alkyl oxygen accelerates the 5-exo-cyclization of the ester radicals because of rotamer populations but retards their 6-exo-cyclization because of the boatlike transition state required to maintain planarity of the ester function in forming a delta-lactone.