The gas-phase reactivity of orthophosphoric acid (HO)(3)P=O with the cation Cu+ has been investigated through the use of mass spectrometry experiments and DFT calculations. Its monomethylated ester (CH3O)(HO)(2)P=O has been also theoretically studied at the same level in order to take into account the possible effects of methylation. Computational results, including structural features, vibrational frequencies, and binding energies, obtained with the hybrid three-parameter B3LYP density functional combined with relativistic ECPs and associated valence basis set, were reported. Final binding energies have been evaluated using extended basis sets. Additional calculations performed with several "all electron" basis sets are also included for comparison. For some of the minima of the [Cu,H3PO4](+) hypersurface, the nature of the different Cu-O bonds as well as the character of the orbitals involved were analyzed by means of the Bader topological analysis and NBO method.