A comparative theoretical study is presented on the formation and decomposition of α-hydroxy-alkylperoxyl radicals, Q(OH)OȮ (Q = RR′C:), important intermediates in the oxidation of several classes of oxygenated organic compounds in atmospheric chemistry, combustion, and liquid-phase autoxidation of hydrocarbons. Detailed potential energy surfaces (PESs) were computed for the HOCH2O2 ̇ ⇌ HO2 ̇ + CH2O reaction and its analogues for the alkyl-substituted RCH(OH)OȮ and R 2C(OH)OȮ and the cyclic cyclo-C6H 10(OH)OȮ. The state-of-the-art ab initio methods G3 and CBS-QB3 and a nearly converged G2M//B3LYP-DFT variant were found to give quasi-identical results. On the basis of the G2M//B3LYP-DFT PES, the kinetics of the ≈15 kcal/mol endothermal α-hydroxy-alkylperoxyl decompositions and of the reverse HO2 ̇ + ketone/aldehyde reactions were evaluated using multiconformer transition state theory. The excellent agreement with the available experimental (kinetic) data validates our methodologies. Contrary to current views, HO2 ̇ is found to react as fast with ketones as with aldehydes. The high forward and reverse rates are shown to lead to a fast Q(OH)-OȮ ⇌ HO2 ̇ + carbonyl quasi-equilibrium. The sizable [Q(OH)OO ̇]/[carbonyl] ratios predicted for formaldehyde, acetone, and cyclo-hexanone at the low temperatures (below 220 K) of the earth's tropopause are shown to result in efficient removal of these carbonyls through fast subsequent Q(OH)OȮ reactions with NO and HO2 ̇. IMAGES model calculations indicate that at the tropical tropopause the HO2 ̇-initiated oxidation of formaldehyde and acetone may account for 30% of the total removal of these major atmospheric carbonyls, thereby also substantially affecting the hydroxyl and hydroperoxyl radical budgets and contributing to the production of formic and acetic acids in the upper troposphere and lower stratosphere. On the other hand, an RRKM-master equation analysis shows that hot α-hydroxy-alkylperoxyls formed by the addition of O2 to C1-, C2-, and C3-α-hydroxy-alkyl radicals will quasi-uniquely fragment to HO2 ̇ plus the carbonyl under all atmospheric conditions.