Oligosaccharide mixtures comprising anti-Xa activities of from 190 IU/mg to 450 IU/mg and anti-IIa activities of less than 0.2 IU/mg; and having constituent oligosaccharides with an average molecular weight of from 1800 to 2400 Daltons, constituent oligosaccharides comprise from 2 to 16 saccharide units, a 4,5-unsaturated uronic acid 2-O-sulfate unit at one end, and comprising at least one ΔIIa-IIs-Is hexasaccharide sequence are described. Processes for preparing the oligosaccharide mixtures, pharmaceutical compositions and articles of manufacture comprising the oligosaccharide mixtures are described. Methods of treatment and prevention comprising administering the oligosaccharide mixtures are described.