Interaction of Al3+ ions with electrostatic complexes of poly(N-diallyldimethylammonium chloride) (PDADMAC) and sodium polymethacrylate (PMANa) in D2O were studied by using H-1, Al-27, Na-23, and Cl-35 NMR, relaxations, and pulsed-gradient stimulated echo (PGSTE) self-diffusion experiments, Al3+ ions are shown to enter the complex structure, substituting up to about 20% of the DADMAC groups in their electrostatic links. The structure of the complex somewhat changes, the free PDADMAC chains being pushed to the outer area of the complex, but remains otherwise intact. This process has an equilibrium nature and larger excess of Al3+ ions does not perceptibly influence its product. It is shown to be an interesting example of a combination of competitive and cooperative interaction: Al3+ ions compete with DADMAC groups in their electrostatic coupling with the polyanion and, at the same time, cooperate with them in the stabilization of the complex.