A set of analytical parameters for GC-MS identification of (RO) ₃P phosphorous acid trialkyl esters has been determined. It Includes standard electron ionization mass spectra, statistically processed ion series spectrum of homologous series, gas chromatographic retention indices on semi-standard non-polar stationary RTX-5 phase, and homologous increments of retention indices. Trialkyl phosphites have been discovered directly in the reaction mixtures of aliphatic alcohols with phosphorous trichloride and without preparative isolation that required the identification of all the constituents of these mixtures. In order to prevent the formation of corresponding (RO) ₂PHO dialkyl phosphonates in the results of Michaelis-Arbuzov rearrangement, the process was carried out by controlling the pH of the reaction mixture with N, N-dimethyl aniline additives. Using joint GC-MS parameters as homologous increments of retention indices (iRI) allowed distinguishing the trialkyl phosphates and the isobaric dialkyl phosphonates (both series belong to the same homologous group y = 12). Besides that, despite the absence of the signals of molecular ions in mass spectra of trialkyl phosphites, the estimation of their molecular masses became possible. The comparison of the average i _RI values as a criterion of chromatographic polarity indicated that trialkyl phosphites are less polar than dialkyl phosphonates. i _RI values for homologues of these series without branches in alkyl substituents were-235 ± 52 and 29 ± 9 respectively.