Safety assessment of nuclear waste disposal includes determination of the possible impact of natural dissolved organic matter on the transport of actinide ions via groundwater into the biosphere. Thereby, much attention is paid to americium as it dominates the radiotoxicity of the nuclear waste after about 300 y and up to about 1000 y (spent fuel) or 100000 y (vitrified reprocessing waste). A trustworthy description of the influence benefits from a sound chemical process understanding of the americium humate complexation and transport processes. A problem in this respect is that studies by TRLFS lead to inconclusive results with respect to the nature of the complexes involved. In the present study the outcome of TRLFS measurements in H2O and D2O, and at room temperature and in liquid nitrogen are compared. It is shown that the Cm3+ ion interchanges between aquo ion (Cm3aq) and humate complex (CmHA) on a time scale of milliseconds in a pH range between 3 and 5.5. Taking this interchange into account, the process can be described in the absence of ternary complexes by the 1:1 stoichiometry formation of one curium humate complex, or a sufficiently narrow distribution of complexes to be represented by one average complex.