High-level ab initio calculations of the barriers, enthalpies, and rate coefficients for the intramolecular cyclization reactions of center dot CH2CH2CH2CH2SeR (R = Me, n-Bu, s-Bu, t-Bu, Bn, Bz, Ph2CH) and center dot CH2CH2CH2CH2CH=CH2 have been performed at the G3(MP2)-RAD level. The calculated rate coefficients show excellent agreement with experiment (to within a factor of 5 or better), although this might be due, in part, to a systematic cancelation of order-of-magnitude errors in the corresponding Arrhenius parameters. The intramolecular cyclizations at selenium were found to be energetically favorable processes that occur with synthetically accessible rate coefficients on the order of 10(4)-10(6) s(-1) at 294 K. These values are largely governed by the stabilization energy of the leaving radical, though with contributions from steric and polar effects.