MP2/6-31+G(d,p) calculations are used to analyze the interaction between CH3X (X = F, Cl, or Br) and hydrogen peroxide (HP). Two stable structures, A and B, are found on each potential energy surface. The A complexes are characterized by a six-membered structure and the B complexes, having a lower stability, by a five-membered structure. In both complexes, the molecules are held together by both OH...X and CH1...O hydrogen bonds. The binding energies range between 2.0 and 3.2 kcal mol(-1) for the A complexes and between 1.5 and 1.7 kcal mol(-1) for the B complexes. The frequency shifts are calculated for the CH1D2D3X isotopomers. Both A and B complexes exhibit simultaneously an elongation of the OH bond and a red shift and an infrared intensity increase of the corresponding OH stretching vibration along with a contraction of the CH1 bond, a blue shift, and an infrared intensity decrease of the CH1 stretching vibration. The interaction of CH3F and CH3Cl with HP also induces a contraction of the external CH2 and CH3 bonds and a blue shift of the corresponding stretching vibrations. The results of an NBO analysis are discussed in terms of the hyperconjugation and rehybridization model. While there is a charge transfer from CH3X to HP in the A complexes. the charge transfer is negligible in the B complexes. Complex formation results in an increase of the occupation of the sigma*(OH) and sigma*(CH1) antibonding orbitals and an increase of the s-character of the corresponding O or C atoms. In contrast, there is a decrease in the occupation of the sigma*(CH2) and sigma*(CH3) orbitals. The n(X) --> sigma*(OH) hyperconjugative energies are equal to similar to10 kcal mol(-1), and the n(O) --> sigma*(CH) hyperconjugative energies range between 1.4 and 2.5 kcal mol(-1) for the A complexes. Our results show that the OH bond lengths are mainly determined by the occupation of the sigma*(OH) orbitals. The CH distances depend on both the occupation of the sigma*(CH) orbitals and the hybridization of the corresponding C atom.