Bioconversions using enzymes immobilized in magnetic supports present significant advantages due to the easy separation of the enzyme from the reaction mixture and the simplicity and low cost of the support preparation. The characterization of the oligosaccharide mixture obtained by the action of b-galactosidase covalently attached, via glutaraldehyde, to a hydrazideeDacronemagnetite composite is presented. The fractionation of the oligosaccharide mixture by high performance liquid chromatography, followed by the analysis of the purified compounds by mass spectrometry and nuclear magnetic resonance spectroscopy permitted the identification of glucose, galactose, lactose and a hexose disaccharide containing a 1/6 linkage. Also, the following GOS were identified: b-D-Galp-(1/6)-b-D-Galp-(1/4)- Glcp, b-D-Galp-(1/4)-[b-D-Galp-(1/6)]-Glcp and b-D-Galp-(1/6)-b-D-Galp-(1/6)-b-D-Galp-(1/4)- Glcp. When GOS yield (26.2%) and kinetics of biotransformation of lactose by the Dacron immobilized b-galactosidase were compared with values obtained for the enzyme immobilized in other magnetic supports, similar behaviour was observed