The infra-red spectra of 51 salts of phosphorus oxy-acids have been examined in the rock-salt region, and general absorption regions characteristic of each class of anion are tabulated. Correlations are suggested for P-F, P-OH, PO₃^2-, P-CH₃ and other structural groupings. Previous frequency assignments have received additional confirmation and the relation of absorption to structure in these compounds is discussed. -- CAL