Rates and thermochemistry for the cyclization of various hydroperoxyalkyl radicals (.)QOOH with up to six carbons to form cyclic ethers plus OH are computed using complete-basis-set (CBS) and density-functional theory (DFT) methods. Effects of mono- and dialkyl substitution a to the OOH group and to the radical center were also studied. Many quantum chemical methods have difficulty accurately predicting peroxide energetics and particular problems with the transition state calculations. The popular B3LYP method underestimates many barrier heights as well as O-O bond strengths by up to 8 kcal/mol. The related BH&HLYP method appears to give more accurate barrier heights predictions than B3LYP, but its thermochemistry is inaccurate and it overestimates the heats of reaction by up to 5 kcal/mol. For the transition states, there are subtle problems even with high-level CBS methods. But from the many calculations, a consistent picture emerges and is compared with the limited existing experimental data. Improved hydrogen-bond increment (HBI) values for beta-hydroperoxyalkyl radicals and ring strain corrections (RSC) for cyclic ethers with 3-, 4-, and 5-membered rings are derived. Generalized rate estimation rules for the decomposition of alkyl-substituted (.)QOOH to form cyclic ethers are presented based on the observed Evans-Polanyi correlation between the computed barrier height and the reaction exothermicity. Issues that must be resolved before these results can be usefully applied in ignition and partial oxidation models are outlined.