[en] By spectrophotometric and potentiometric methods it is shown that trimers are formed in media of high ionic strength (3 M NaClO₄) and total concentrations of hexavalent actinides higher than 10^-3 M. The equilibrium constants for the reactions 3 MO₂(CO₃)₃^4- ↔ (MO₂)₃(CO₃)₆^6- + 3 CO₃^2- are lg K(U)=-11.3±0.1; lg K(NP)=-10.1±0.1; lg K(Pu)=-7.4±0.2. It is demonstrated that one of the cations of the trimer can be exchanged with another actinide cation: the equilibrium constants for the reactions 2 UO₂(CO₃)₃^4- + MO₂(CO₃)₃^4- ↔ (UO₂)₂MO₂(CO₃)₆^6- + 3 CO₃^3- are lg K=-11.3±0.1, -10.0±0.2 and -8.8 respectively for M=U, Np, Pu. Thus, polynuclear complexes can be efficient solution ''carriers'' for other hexavalent actinides in waste disposal. Some properties of the solid phases MO₂CO₃(s) are discussed. Experimental studies of chemical equilibria of americium (III, IV) are reviewed: in carbonate media americium(III) species are AmCO₃⁺, Am(CO₃)₂^-, Am(CO₃)₃^3-, (Am₂(CO₃)₃)₆ and (NaAm(CO₃)₂)_s; for americium-(IV) lg β₅ ≅ 40. In 3 M NaClO₄ solubility measurements of neptunium(V) show that lg β₁=5.09, lg β₂=8.15, lg β₃=10.46, lg Ks(NaNpO₂CO₃)=10.56 and lg Ks(Na₃NpO₂(CO₃)₂)=12.44; ionic strength corrections are proposed on the basis of these results. (orig.)