Abstract To study the chemical nature of the acid catalysis of amides oftrivalent-phosphorus acids (ATPA) alcoholysis the most important procedural problems have been solved. Simple and efficacious techniques for the purification of original ATPA from amine hydrohalides have been worked out. A precise and reliable way has been chosen to control the residual amount of amine hydrohalides in ATPA having been subjected to the various purification procedures. The nature of the dependence of alcoholysis rate constants for ATPA of different structures on the aminehydrohalide concentration has been established. The general acid catalysis was established using the Brønsted equation, so that the process includes the formation of a catalytic H-complex incorporating substrate and catalyst on the whole. Amides P(III) of different types treated with anhydrous hydroborofluoric acid readily form the corresponding P-protonated salts. The conversions of corresponding products of complete protonation have been investigated. The structure of P-protonated salt 23 has been studied by means of X-ray analysis.