The invention relates to a process for preparing isomalto-oligosaccharides with elongated chain length. Isomalto-oligosaccharides are directly converted to isomalto-oligosaccharides with elongated chain length in the presence of glucansucrase. Said products can be applied in food, feed, beverages, cosmetics or pharmaceutical products and are particularly useful as slow or non-digestible oligosaccharides, low calorie providers, prebiotics, mineral absorption promoting agents, non-cariogenic agents and/or low glycemic index regulating syrups.