The di-, tri- and tetrasaccharides formed during Trichoderma reesei endo-β-d-mannanase treatment of pine kraft pulp were studied. The oligosaccharides in the hydrolysate were fractionated using size-exclusion, anion exchange and activated carbon chromatography. The primary sequence of the purified oligomers was determined by two-dimensional NMR techniques. The T. reesei mannanase cleaves the β-1,4-glycosidic linkage of d-mannosyl residues attached either to d-mannose or d-glucose. The d-mannosyl residue may also be substituted by a d-galactosyl group. The main disaccharide produced was mannobiose, but a significant amount of 4-O-β-d-glucopyranosyl-d-mannopyranose (GlcMan) was also produced. After extensive hydrolysis the main trisaccharides produced were 4-O-β-d-mannopyranosyl-[6-O-α-galactopyranosyl]-d-mannopyranose (Gal1Man2) and 4-O-β-d-glucopyranosyl-4-O-β-d-glucopyranosyl-d-mannopyranose (Glc2Man). Some mannotriose 4-O-β-d-glucopyranosyl-4-O-β-d-mannopyranosyl1-d-mannopyranose (GlcMan2) and 4-O-β-d-glucopyranosyl-[6-O-α-galactopyranosyl]-d-mannopyranose (Gal1GlcMan) were also detected in the hydrolysate. The structures of two tetrasaccharides were studied. They appeared to be 4-O-β-d-glucopyranosyl-4-O-β-d-glucopyranosyl-4-O-β-d-glucopyranosyl-d-mannopyranose (Glc3Man) and 4-O-gb-d-glucopyranosyl-4-O-β-d-mannopyranosyl-4-O-β-d-glucopyranosyl-d-mannopyranose (GlcManGlcMan). According to the results obtained, the galactoglucomannan in pine contains regions in which two or three glucose units are linked together, which further means that it may contain regions with several successive mannose residues. The galactose side groups were found to be attached only to mannose.