The present invention concerns a composition comprising complexes of cationic chitosan oligomers derived from the cationic polysaccharide chitosan, wherein said cationic oligomers contain a weight fraction of less than 20% of oligomers with a Degree of Polymerization (DP) 50, and a nucleic acid. These compositions comprising well-defined cationic chitosan oligomers having a certain distribution of chain lengths, and nucleic acid are advantageous to achieve delivery of the nucleic acid into cells of a selected tissue, and to obtain in vivo expression of the desired molecules encoded for by the nucleic acid.