The complexation of lanthanide(III) (lanthanum, europium, and lutetium) and americium(III) by four tridentate nitrogen-donor ligands was investigated in homogeneous methanol/water solutions by using structural and thermodynamic approaches. The stoichiometry and inner-sphere hydration state of the europium complexes formed were determined by time-resolved laser-induced fluorimetry. The stability constants and in some cases the thermodynamic parameters were measured by ultraviolet-visible spectrophotometry and the van't Hoff method. The comparison of the stability constant of americium complexes with lanthanide complexes shows better stability for the actinide species. The strong affinity for these ligands toward Am3+ is confirmed by the formation of higher complexes, especially in the case of 2,6-bis-(5,6-dimethyl-1,2,4-triazin-3-yl)-pyridine (MeBTP), the only ligand able to form a 1:3 complex. The influence of the solvent composition on the complexation thermodynamics highlights the effect of the solvent reorganization on the reaction.