[en] By using spectrophotometric and potentiometric methods it is shown that, in media of high ionic strength (NaClO₄ 3M) and total concentrations of hexavalent actinoids higher than 10^-3M, trimers are formed. The equilibrium constants for the reactions 3MO₂(CO₃)₃^4- reversible (MO₂)₃(CO₃)₆^6- + 3 CO₃^2- are log K (U) = -11,3; log K (Np) = -10,1 and log K (Pu) = -7,4. It is demonstrated that one of the cations of the trimer can be exchanged with another actinoid cation: the equilibrium constants for the reactions 2UO₂ (CO₃)₃^4- + MO₂ (CO₃^4- reversible (UO₂)₂ MO₂ (CO₃)₆^6- + 3CO₃^3- are log K = -11,3; -10,0 and -8,8 respectively for M: U, Np, Pu. Then, polynuclear complexes can be efficient solution ''carriers'' for other hexavalent actinides in a 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 Am(III) species are Am CO₃⁺, Am (CO₃)₂^- Am (CO₃)₃^3-, (Am₂ CO₃)sub(s) and (NaAm(CO₃)₂)sub(s); for Am(IV), log B₅ approximately equal to 40. In NaClO₄ (3M) medium solubility measurements of Np(V) show that logβ₁ = 5,09, logβ₂ = 8,15, logβ₃ = 10,46, log Ks(Na Np O₂ CO₃) = 10,56 and log Ks (Na₃ NpO₂(CO₃)₂) = 12,44; ionic strength corrections are proposed