The characterization of the main composite sugars of commercial gelling red algae galactans (agarose, iota and kappa carrageenans) by methanolysis and separation of the methyl glycosides produced by high performance liquid chromatography is described. The methanolysis (methanolic hydrochloric acid strength, temperature, and reaction time) was optimized in order to release monosaccharides in near quantitative yield. The results were compared to those obtained by (1) gas chromatography of the alditol acetates of the neutral sugars released by acid hydrolysis and (2) specific colorimetric determination of the acid-labile 3,6-anhydrogalactosyl residue. Conditions such as methanolic 0.125 M HCl for 1 h at 85 degrees C were sufficient to release all of the galactosidic and 3,6-anhydrogalactosidic bonds for iota carrageenan without apparent degradation of the anhydrogalactosyl unit. However, with the same conditions, the yields of 3,6-anhydrogalactosyl residues were 80 and 70% for kappa carrageenan and agarose, respectively. These yields were not improved by stronger conditions. At the opposite extreme, under very mild methanolysis conditions such as methanolic 0.01 M HCl at 100 degrees C for 1 h, agars and gelling carrageenans were well differentiated by the respective determination of agarobiose- and carrabiose-dimethyl acetal which are well-separated on octadecyl reversed phase HPLC columns with water as eluent.