Sodium alginates obtained by alkaline extraction of Lessonia flavicans, Desmarestia ligulata and Desmarestia distans (Phaeophyta) from southern Chile were partially hydrolyzed with HCl. Each alginate gave three fractions that were characterized using FT-IR spectroscopy. The fractions soluble in 0.3M HCl presented in the fingerprint region four vibrations at around 960, 911, 890 and 815 cm(-1) that were assigned to heteropolymeric blocks. The fractions soluble at pH 2.85 showed bands at around 948, 888 and 820 cm(-1) attributed to homopolymannuronic acid blocks, the first band is resolved in the second-derivative spectra into two bands at 951 and 936 cm(-1). The fractions insoluble at pH 2.85 presented four bands at around 947, 903, 812 and 781 cm(-1), which were assigned to homopolyguluronic acid blocks. For some samples, the second derivative FT-IR spectra showed new bands indicating the presence of other structures, in low proportions. Structures deduced by FT-IR spectroscopy were corroborated by solution (1)H and (13)C NMR spectroscopy. Two-dimensional spectra were collected to confirm the fine structure of the hetero- and homopolymeric fractions. A geometrically optimized model for the disaccharide alpha-l-gulopyranuronosyl-(1-->4)-alpha-l-gulopyranuronic acid was calculated using density functional theory; good agreement was obtained between its corresponding calculated vibrations and the experimental bands assigned to homopolyguluronic acid blocks.