The homologous series of dialkyl phosphonates (CnH2n + 1O)2PH=O] (2 ≤ n ≤ 9) (dialkyl phosphites is their commonly used trivial name) was characterized by electron ionization mass spectra and retention indices (RIs) on an RTX-5 standard nonpolar stationary phase. It was established that, in the presence of even one chiral center in the alkyl fragments (for example, in sec-alkanol esters), dialkyl phosphonates were detected in the form of several chromatographic signals due to diastereomers with differences in the retention indices from 4 to 21. The four-coordinated phosphorus atom is the second chiral center. The homologous increments of the retention indices of di-n-alkyl phosphonates (iRI ± sRI = 29 ± 9) and esters having one (–4 ± 10) and two (–61 ± 14) branching points in the carbon skeletons of alkyl fragments were evaluated. These values, which were calculated based on data for a limited number of homologues, make it possible to characterize any compounds from the test series. It was found that the relationship М ≈ 0.14(RI – iRI) + y makes it possible to uniquely determine the molecular weights of analytes according to chromatographic data under the condition of rounding the results to the nearest value of M comparable with y = 12 to the modulus 14 [М ≡ 12(mod14)].