Dimethyl methylphosphonate (DMMP), an important flame retardant in lithium-ion batteries, has been studied theoretically. The energy, enthalpy, and Gibbs free energy of DMMP and its protonated form (DMMP-H+) have been calculated using the high-level ab initio methods G3(MP2). G3(MP2)//B3LYP, G3, G3//B3-LYP, and CBS-QB3. All calculated proton affinities showed good agreement with experiment (within 1.5%), with the best values being obtained with G3MP2. At this level of theory, the calculated proton affinity of DMMP is 895 kJ . mol(-1). The ionization potential (9.94 eV) was calculated using the related procedure G3(MP2)-RAD, and also showed excellent with experiment (0.6%). Hydrogen bonding in DMMP-H+ hits also been studied. (C) 2008 Elsevier Ltd. All rights reserved.