The present invention describes a process for preparing acetals of polysaccharides, oligosaccharides or disaccharides by dissolving these in at least one ionic liquid and reacting them with a vinyl ether. The acetalated polysaccharides or oligosaccharides obtained in this way can be crosslinked by treatment with acid. The present invention also provides acetals of polysaccharides, oligosaccharides or disaccharides and also crosslinked polysaccharides or oligosaccharides.