Rate constants for cyclization of the 1-methoxy-6,6-diphenyl-5-hexenyl radical (2a) and the 1-methoxy-7,7-diphenyl-6-heptenyl radical (2b) were measured directly. The 5-exo cyclization of 2a in THF and in CH3CN displays no solvent effect; it is described by log(k(c).s) = 9.3 - 2.3/2.3RT, and the rate constant for the cyclization at 25 degrees C is 3.8 x 10(7) s(-1). Radical 2b cyclizes in a 6-exo fashion with rate constants approximately 2 orders of magnitude smaller than those of 2a; log (k(c).s) = 8.8 - 4.9/2.3RT, and k(c) at 25 degrees C is 1.5 x 10(5) s(-1). The kinetics of bimolecular hydrogen atom transfer trapping of 2a by t-BuSH (log (k(T).M.s)j = 8.4 - 2.1/2.3RT) and 2b by Bu(3)SnH (log (k(T).M.s) = 8.4 - 3.8/2.3RT) also were measured. The polarity-matched thiol reacts with 2a with rate constants similar to those for reaction with a primary alkyl radical, but the polarity-mismatched Bu(3)SnH reacts with 2b about an order of magnitude less rapidly than with an alkyl radical.