Ab initio and density functional calculations predict that intramolecular homolytic substitution reactions of oxyacyl radicals at the selenium atom in ω-alkylseleno-substituted radicals proceed via mechanisms that do not involve hypervalent intermediates. When the leaving radical is tert-butyl, energy barriers (ΔG‡) for these reactions range from 27.1 (G3(MP2)-RAD) kJ mol−1 for the formation of the five-membered cyclic selenocarbonate (6) to 41.5 kJ mol−1 for the six-membered selenocarbonate (8). G3(MP2)-RAD calculations predict rate coefficients in the order of 105–108 s−1 and 102–105 s−1 for the formation of 6 and 8, respectively, at 298.15 K in the gas phase. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 44: 51–58, 2012