Absorption and fluorescence spectroscopic measurements, as well as isothermal calorimetric titrations showed that the biexponential decay kinetics of protonated ellipticine (EH+) fluorescence in water originates from dimerization. Due to the high equilibrium constant for the association of two EH+ and the intense fluorescence of the dimer, deviation from the exponential emission intensity decay commences below micromolar concentrations. Dimerization must be taken into account when EH+ concentration is determined by spectrophotometry, and when EH+ binding to substrates are studied. The molar absorption coefficients of the monomer and dimer were determined. (C) 2015 Elsevier B.V. All rights reserved.