The proton ionisation constants of cysteine methyl ester have been determined at 25°, 37°, and 50° and I= 0·1M(KCl). Values of ΔH° and ΔS°₂₉₈ for the various ionisation equilibria have been calculated. Stability constants have been determined potentiometrically at 25° and I= 0·1M for complexes present in aqueous solutions of cysteine methyl ester and the bivalent ions of nickel, zinc, cadmium, mercury, and lead. In the case of nickel(II) the complexes NiL⁺, NiL₂, NiL₃^–, Ni₄L₆^2–, NiHL²⁺, and NiHL₂⁺ occur. Confirmatory evidence for the existence of polynuclear species in this system has been obtained spectrophotometrically. Polynuclear complexes do not occur with zinc(II) and lead(II), but protonated species (ZnHL²⁺, ZnHL₂⁺) and hydrolysed species [ZnL(OH)] occur with zinc in addition to simple mononuclear complexes. Protonated complexes also occur with lead(II). A polynuclear complex Cd₂L₃⁺ occurs with cadmium, in addition to the simple mono- and bis-species. In the case of mercury(II) it is suggested that the complex HgL⁺ is completely formed at the commencement of pH-tritrations at a 1 : 2 metal-to-ligand ratio (i.e., log β₁ is very large). Using this assumption the titration data can be satisfactorily described by the set of complexes HgL₂, Hg₂L₃⁺, and Hg₃L₄²⁺. A number of solid complexes have been isolated and their infrared spectra studied. Of particular interest is the isolation of a number of trinuclear complexes of the type [M{Ni[SCH₂CH(NH₂)COCH₃]₂}₂]MCl₄ where M = Zn(II), Cd(II), and Hg(II).