The confirmation of the correspondence of the detected forms of analytes to their molecular structures seems to be one of the principal problems of analytical chemistry. The previous analysis of the series of drugs by reversed-phase HPLC allowed revealing that the retention regularities of the complex polyfunctional organic compounds containing sulfonamide groups -SO2-N< differ from those of other compounds. It was explained by reversible formation of their hydrate forms. To verify this proposition, three model N-substituted arylsulfonamides were synthesized, namely N,N-diethyl- (I), N-allyl- (II), and N-phenyl (III). The consideration of the dependencies of these analytes net retention times ( t R) in reverse-phase HPLC on the content of organic solvent ( C , methanol) in an eluent did not permit us to reveal any regularities or anomalies due to the non-linearity of such dependencies. However, to achieve this, the recurrent approximation was recommended, t R( C + D C ) = at R( C ) + b (*), where D C = const - constant variations of concentration of methanol (5% in our case), а and b - coefficients calculated by Least Squares Method. If the chemical origin of the analytes at the outlet of the chromatographic column remained the same within concentration range С min < С < С max, dependencies (*) were linear with correlation coefficients R > 0.999. Yet, if additional chemical transformations of analytes took place, namely reversible formation of hydrates (or variations of their composition) due to the presence of water in an eluent, it led to the deviations of recurrent dependencies (*) from linearity. Three sulfonamides under characterization corresponded to different kinds of such deviations: non-linearity within the whole range С min < С < С max (amide I), presence of two linear dependencies instead of one (amide II), and linearity within the whole range С min < С < С max (amide III). First two cases corresponded to the interconversion of anhydrous forms and hydrates, or (less probable) different hydrates, while the latter meant the existence of analyte in the single form (probably hydrate). Thus, the analysis of model compounds confirmed that the presence of the polar functionality -SO2-N< was just the reason of hydrate formation in water solutions.