Disclosed is a method for purifying sialylated oligosaccharides from a fermentation broth, cell-lysate or biocatalytic reaction mixture for obtaining high amounts of desired sialylated oligosaccharides in high purity. The method is particular suitable for the large-scale economic purification of sialylated human milk oligosaccharides (such as 3' -sialyllactose, 6' -sialyllactose or sialylated lacto-N-tetraose derivatives) from microbial fermentation, using recombinant bacterial cells or yeast cells. The obtained material is of high purity and can be used for food or medical application such like medical nutrition products, infant formula, dietary supplements, general nutrition products (e.g. dairy drinks).