The i.r. and Raman spectra of Na 3HP 2O 7·9H 2O, Na 3HP 2O 7·H 2O, Na 3HP 2O 7, Cs 3HP 2O 7 and their deuterated derivatives have been carried out between 4000 and 200 cm −1. All the internal vibrations were assigned; in all these salts the HP 2O 3− 7 anions are linked by strong hydrogen bonds forming infinite chains. Two types of water molecules were spectroscopically determined in Na 3HP 2O 7·9H 2O. The total and partial dehydration of this salt affect the crystal packing but do not really affect the strength of the hydrogen bond which is similar in all the salts studied. Brown's correlation (ν as POP—ν s POP) = f ( POP ) established for neutral pyrophosphates has been extended to hydrogenopyrophosphates. For the first time a linear correlation δPOP = f ( POP ) has been established.