The complexation of Cm(III) and Eu(III) with 2,6-bis(5,6-di(sulfophenyl)-1,2,4-triazin-3-yl)pyridine (aq-BTP) is studied in water at pH 3.0 applying time-resolved laser fluorescence spectroscopy. With increasing ligand concentration [M(H₂O)_9−3n(aq-BTP)_n] (M = Cm(III)/Eu(III), n = 1, 2, 3) complex species are spectroscopically identified. The conditional stability constants of the M(III) 1:3 complex species with aq-BTP are log β₀₃ = 12.2 for Cm(III) and log β₀₃ = 10.2 for Eu(III). The complexation reaction is enthalpy- and entropy-driven for both metal ions, while the enthalpy change ΔH₀₃ is 9.7 kJ mol⁻¹ more negative for Cm(III); changes in ΔS₀₃ are marginal. The difference in ΔG₀₃ of −12.7 kJ mol⁻¹ between the formation of the [M(aq-BTP)₃] complexes agrees with aq-BTP's selectivity in liquid–liquid extraction studies.