The stability constants for Mn+-tricine binary complexes (1:1 and 1:2), where Mn+ = Th4+, Ce3+, La3+, and UO22+, have been determined pH-metrically in aqueous media at 25 degrees C and in the presence of 0.10 mol dm(-3) (KNO3). The binary Th-tricine (1:1) complex was studied in the presence of different organic solvents as well as in the presence of different ratios of dioxane and at different temperatures. The calculated stability constants have been discussed in terms of the nature of both the metal ion and tricine. It is suggested that tricine may act as a tridentate as well as a bidentate ligand. Solvent basicity and proportion in addition to the differences in stabilization of the free base by hydrogen bonding with solvent molecules as well as the electrostatic effect are the major factors responsible for the observed high stability of the Th-tricine binary complex. The thermodynamic parameters (Delta H, Delta G, and Delta S) for the formation of 1:1 Tn-tricine complex in the presence of 40% (w/w) dioxane at different temperatures (5-45 degrees C) were calculated.