The invention relates to an oligosaccharide having a weight average molecular weight (Mw) of less than 3,000 Daltons, wherein said oligosaccharide comprises functional aldehyde groups and has a dimer saccharide content of 2.9 % by weight or less, based on the total weight of the hydrogenated oligosaccharide. The invention also relates to a process for preparing the oligosaccharide, which process comprises the steps of: hydrolysing a polysaccharide so as to reduce its molecular weight, wherein the resulting oligosaccharide comprises functional aldehyde groups, and fractioning the oligosaccharide according to molecular weight, so that the purified fraction has a weight average molecular weight equal to or less than 3,000 Daltons. The procedure of purification is performed by one or more membrane processes, having a cut-off value between 340 and 800 Daltons, which procedure is continued until the content of dimer saccharide in the purified fraction of oligosaccharide has been reduced to 2.9 % by weight or less, based on the total weight of the hydrogenated oligosaccharide.
