The phenol-sulfuric acid reaction (PSAR) is the major colorimetric method to quantitatively determine the presence of carbohydrates in solutions in biochemical and bioorganic experiments. Here, the spectral characteristics of the PSAR products formed from the lipopolysaccharides (LPSs) and glycosylated flagellin of Azospirillum bacteria were examined and they were found to coincide with the theoretical spectra predicted taking account of the monosaccharide contents in these glycoconjugates. The spectra 350-600 nm of the PSAR products of various monosaccharides were used for the calculation of the proportions of carbohydrates in the polysaccharide-containing macromolecules of bacteria of the genus Azospirillum. For LPS preparations, these proportions ranged from 32% for LPS of Azospirillum brasilense S27 to 97% for LPS of Azospirillum brasilense SR88. The mass fraction of carbohydrates in glycosylated flagellin of the polar flagellum from Azospirillum brasilense Sp7 was determined as ~43%. And the proportion of carbohydrates in the capsule polysaccharides of Azospirillum lipoferum Sp59b was 61% versus 49% in the composition of LPS of the same strain. Also, it was demonstrated that the spectra of the PSAR products can be used to estimate the average ratio between the mass fractions of the carbohydrate-containing parts of the LPS: O-specific polysaccharide and core. For heteropolysaccharides formed from regular repeating units containing different monosaccharide residues, the spectral peculiarities of the PSAR products may serve as unique characteristics, and the reaction itself could be used to fulfill diverse tasks in the study of carbohydrate-containing preparations, apart from the traditional detection of carbohydrates in samples.