The infra-red spectra of 92 salts of phosphorous oxy-acids have been examined in the rock-salt region. Characteristic frequencies are suggested for P-N and P-O-P linkages. Confirmation of a number of frequency assignments made by other workers has been obtained. The absorption frequencies are found to be largely independent of the positive ion, but show considerable variation with the nature of the anion. General absorption bands characteristic of each class of negative ion are listed and their possible use in analysis is indicated. Effects of crystal and molecular structure on the absorption spectra are discussed, and some useful structural information is derived. -- CCI