The existence of a series of organic peroxy radical-water complexes [CH3O2 center dot H2O (methyl peroxy); CH3CH2O2 center dot H2O (ethyl peroxy); CH3C(O)O-2 center dot H2O (acetyl peroxy); CH3C(O)CH2O2 center dot H2O (acetonyl peroxy); CH2-(OH)O-2 center dot H2O (hydroxyl methyl peroxy); CH2(OH)CH2O2 center dot H2O (2-hydroxy ethyl peroxy); CH2(F)O-2 center dot H2O (fluoro methyl peroxy); CH2(F)CH2O2 center dot H2O (2-fluoro ethyl peroxy)] is evaluated using high level ab initio calculations. A wide range of binding energies is predicted for these complexes, in which the difference in binding energies can be explained by examination of the composition of the R group attached to the peroxy moiety. The general trend in binding energies has been determined to be as follows: fluorine;approximate to alkyl < carbonyl < alcohol. The weakest bound complex, CH3O2 center dot H2O, is calculated to be bound by 2.3 kcal mol(-1), and the strongest, the CH2(OH)O-2 center dot H2O complex, is bound by 5.1 kcal mol(-1). The binding energy of the peroxy radical-water complexes which contain carbonyl and alcohol groups indicates that these complexes may perturb the kinetics and product branching ratios of reactions involving these complexes.