A convenient synthesis and a full characterization of the strong acid bis[bis(pentafluoroethyl)phosphinyl]imide and some of its salts M+N[(C2F5)(2)P(O)](2)(-), M = Na, K, Cs, Ag, Me4N, are presented. Their thermal (mp, T-dec.) and spectroscopic (IR, Raman, NMR) properties are discussed. A single crystal structure of [Me4N][N{P(O)(C2F5)(2)}(2)] has been obtained, and the structural parameters of the anion are compared with the results of quantum-chemical calculations. The observed properties are comparable to those of bis((trifluoromethyl)sulfonyl)imide and their derivatives.