The aim of this study was studying the effect of acidic hydrolysis conditions on the molecular parameters of galactomannan, such as weight average degree of polymerization (DPw), weight average molecular weight (Mw), product yield, depolymerization reaction rate, and mannose/galactose (Man/Gal) ratio in the presence of hydrochloric acid and obtaining low molecular weight oligogalactomannans soluble in water. A preparing possibility low molecular weight galactomannans by acid hydrolysis of guar galactomannan (DPw=1160, Mw=563.8 kDa, Man/Gal=1.89) in the presence of hydrochloric acid was studied. The studies were carried out in hydrochloric acid solutions with various concentrations (0.05–1.0 mol/L) for different times (10–240 min) at a constant temperature (80 °C). In the course of the studies, effects of the reaction conditions on the molecular parameters of the product obtained were analyzed. The conducted reactions yielded low molecular weight galactomannans and oligogalactomannans with different molecular parameters (DPw=6–1021, Mw=2.6–505.1 kDa, Man/Gal=1.90–3.19) and yields (2.8–97.1%). Based on the results obtained, the optimal reaction conditions (HCl 0.1 mol/L, 240 min, 80 °C) were supposed for the preparation of galactomannan oligosaccharides. The structural studies of the products indicated that products with reduced galactose residues could be prepared by this method.