The interaction of bivalent [Cu(II), Ni(II), Zn(II), Co(II), Mg(II), Ca(II)] and trivalent [La(III), Pr(III), Nd(III), Sm(III), Gd(III), Dy(III)] metal ions with thymidine both in binary and ternary systems have been investigated by potentiometric pH measurements at 35°C and 0·10M (KNO₂) ionic strength. The secondary ligands used in this investigation were glycine, histidine, histamine,o-phenanthroline and α, α’-bipyridyl. The various stability constants pertaining to the above interactions were computed and compared. The influence of pH on the concentrations of several species present in aqueous solution, has been assessed. The effects of (i) charge on the metal ions, and (ii) the presence of a methyl group on the pyrimidine ring, on the stabilities of complexes have been assessed. A comparison with the corresponding constants of uridine has been made. Thymidine forms more stable complexes than uridine.