The electronic structure of methylene peroxide is a balance between zwitterionic and biradical valence structures. In this work we study how this balance is changed by an aqueous solvent, and what effect this has on the nuclear structure. A combined quantum chemical statistical mechanical method with an explicit representation of the solvent is used. The multiconfigurational quantum chemical method used, is very similar in quality to the complete active space self-consistent-field (CASSCF) method. The optimized structure of methylene peroxide in aqueous solution is reported along with properties of the solvation such as radial distribution functions for the surrounding water. The solvent increases the contribution from the zwitterionic form in the wave function. Moreover the lowest energy absorption of methylene peroxide in water is computed. Our best estimate is 2.94 eV. (C) 2009 Elsevier B.V. All rights reserved.