The complexation of DNA with a dendrimer with the successive addition of the dendrimer solution to the solution of DNA leads to the appearance and accumulation of compact particles with a diameter of Dh=100 10 nm that coexist with particles whose dimensions coincide with those of free DNA molecules (Dh = 450 50 nm). The fraction of large particles decreases regularly; moreover, when the equimolar ratio of charged groups of interacting components is achieved, these particles disappear. The coexistence of practically free DNA molecules and a compact soluble negatively charged complex is confirmed by a sedimentation assay. Further addition of the dendrimer up to a certain ratio of charged groups of the components, much higher than unity, brings about phase separation, while at a large excess of charge groups of the dendrimer (above fivefold), the system turns out to be fully single-phase and composed of compact (110 10 nm) cationic soluble complexes. Owing to the positive charge and small size of complex part