To evaluate the rationality and separation conditions of alkyl esters of phosphorous P(III) and P(V) acids using reversed phase HPLC with mass spectrometric detection, the analyses of reaction mixtures of two 1-alkanols (1-butanol and 1-heptanol) with PCl3 in the presence of N,N-dimethyl aniline (for pH correction) have been carried out. In accordance with the previous results, these mixtures preferably contain trialkyl phosphites (RO)3P and dialkyl phosphonates (RO)2PH=O. However, the results of HPLC-MS analyses indicate the prevalence of oxidation products P(III) ® P(V) in the composition of these mixtures, namely trialkyl phosphates (RO)3OP. In accordance with this scheme, the appearance of diheptyl phosphate (C7H15O)2P(=O)-OH is caused by the oxidation of diheptyl phosphonate. Possible reasons of such discrepancies in the composition of reaction mixtures are discussed. One of them may be oxidation of P(III) derivatives by air oxygen during storage, or, tentatively, due to electrochemical reactions during electrospray ionization. The reaction mixture of 1-heptanol with PCl3 contains two components with the same molecular weights ( M = 294) and identical mass spectra, but strongly different retention parameters ( t R 5.4 and 16.0 min). The second of them is diheptyl phosphonate. To explain the appearance the first of them, existence of more hydrophilic prototropic tautomer (C7H15O)P-OH (contains hydroxyl group in the molecule) was proposed. Comparing the analytical results obtained in positive and negative modes of detection indicates that the first of them seems to be more effective for detection of alkyl esters of P(III) and P(V) phosphorous acids.