Norbadione A (NbA) is a mushroom pigment, which is assumed to be involved in (137)Cs accumulation all over Europe during the Chernobyl nuclear accident. NbA bears seven acid-base functional groups, among which are two enolic and two carboxylic acid moieties. This work deals with complex formation of Cs(+) and NbA in ethanol, ethanol/water (9:1) (M1), and water with, when required, the support of two Cs(+) ionophore probes, calix[4]arene-bis(crown-6-ether)dioxycoumarine (A1) and its tetrasuslfonated form (A2). In ethanol, two Cs(+) complexes are formed, with the affinity constants K(1EtOH) = (1.1 ± 0.25) × 10(5) and K(2EtOH) = (2.1 ± 0.4) × 10(3). In M1, a single Cs(+) complex occurs when only the enols are deprotonated, whereas a bicomplex is formed when both enols and carboxylic acids are deprotonated: K(1M1) = (1.5 ± 0.3) × 10(5) and K(2M1) = (4 ± 2) × 10(3). These data are confirmed by stopped-flow and T-jump kinetics. In ethanol, a fast Cs(+) exchange occurs between NbA and A1: direct rate constant, k(1) = (3.1 ± 0.1) × 10(7) M(-1) s(-1); reverse rate constant k(-1) = (2.8 ± 1) × 10(5) M(-1) s(-1); and Cs(+) exchange constant, K(1Exchange) = (9 ± 4) × 10(-3). In M1, the quenching of A2 fluorescence by NbA is used to determine the kinetics of complex formation with Cs(+): k(2) = (1.8 ± 0.4) × 10(9) M(-1) s(-1); k(-2) = (1.80 ± 0.15) × 10(4) s(-1); and K(1M1) = (1.5 ± 0.5) × 10(5). The affinity of NbA for Cs(+) is probably the result of the particular structure in which the two pulvinic acid arms adopt a conformation that forms two complexation sites composed of the two enolates and/or the two carboxylates. This renders the efficiency in Cs(+) uptake comparable to that of some calixarenes or crown ethers.