Abstract: The invention relates to nanoparticles containing complexes constituted by nucleic acids and cationic copolymers containing the recurring structural units of formulae (Ia) and (Ib) wherein R1 and R6 independently represent hydrogen, alkyl or —COOR9, R2 and R7 independently represent hydrogen or alkyl, R3 is selected from the group consisting of —O—R10—, —COO—R10, —CONH—R10- or —R10—, R4 represents hydrogen, alkyl, cycloalkyl, aryl, aralkyl or alkylaryl, R5 represents hydrogen, alkyl, cycloalkyl, aryl, aralkyl, alkylaryl or —(alkylene-NH—)malkyl, or R4 and R5 together with the nitrogen atom they have in common form a heterocyclic ring, R8 is selected from the group consisting of —O—R11, —COO—R11, —CONH—R11 or —R11, R9 and R11 independently represent hydrogen or a monovalent organic residue, R10 represents a bivalent organic residue, and m is an integer from 1 to 5, with the proviso that the nanoparticles have a diameter (z-average) of less than or equal to 900 nm as determined by dynamic light scattering a