Matrix isolation infrared spectroscopy has been combined with theoretical calculations for the characterization of the 1:1 hydrogen-bonded complex between H2O and dimethyl methylphosphonate (DMMP). The symmetric O-H stretching mode was observed to shift 203 cm(-1) to lower energy upon hydrogen bond formation, while a 32 cm(-1) blue shift was noted for the H-O-H bending mode of the H2O subunit in the complex. These values compare extremely well with the (unscaled) shifts of -203 and +32 cm(-1), respectively, that were calculated theoretically at the MP2/6-31+G** level. Additional perturbed modes of the DMMP subunit were observed, shifted relative to the parent band position. The greatest perturbation was to the P=O stretching mode near 1270 cm(-1), where a shift of -17 cm(-1) was observed (-21 cm(-1) calculated theoretically). This suggests that the site of hydrogen bonding in the complex is at the P=O group, in agreement with theoretical calculations. The binding energy DeltaEdegrees for the 1:1 complex was calculated to be -7.7 kcal/mol.