Abstract G3(MP2)-RAD calculations provide activation energies for intramolecular homolytic substitution in the 4-(alkylselenoxo)butyl and 4-(alkylselendioxo)butyl radicals ranging from 21�39 kJ mol�1, and 143�170 kJ mol�1 for the selenoxide and selenone, respectively. Arrhenius data translate into rate constants for ring-closure of 1.5�105�2.5�108 s�1 (80°) for the selenoxides, and 5.4�10�14�5.1�10�11 s�1 (80°) for the corresponding selenones. {NBO} analyses show alkyl radicals are electrophilic during homolytic substitution at selenoxide selenium. The dominant orbital interaction in the transition state is worth 2413 kJ mol�1 and involves the {SOMO} and the lone-pair of electrons on selenium. The corresponding selenones are calculated to ring-close through transition states in which alkyl radicals are nucleophilic, but involve weak (SOMO--> �* and SOMO--> �*) interactions. Consequently, this chemistry is not viable for selenones because of the lack of lone-pairs of electrons on the chalcogen.