Cyclic voltammetry (CV) and square-wave voltammetry (SWV) techniques have been used to study the binary complexes of Co(II), Ni(II) and Cu(II) with sulfamethazine (SMZ) at a static mercury drop electrode (SMDE) in 0.04 M Britton-Robinson (B-R) buffer. SMZ gave three peaks at 0.01, -1.32 and -1.55 V. Cu(II)-SMZ complex was recognized by a cathodic peak at -0.38 V. Ni(II)-SMZ complex was reduced at more positive potential (-0.77 V) than that of the hydrated Ni(II) ions (-1.08 V). Co(II)-SMZ complex is investigated at pH 7 and 8. The Co(II) complex at pH 7 is appeared as a shoulder at -1.19 V, whereas this peak becomes a well-separated form at pH 8. The study indicated that the SMZ serves as a catalyst in the reduction of Co(II) and Ni(II) ions. From electronic spectra data of the complexes, their stoichiometries of 1 : 2 (metal-ligand) in aqueous medium are determined. The stability constants of the complexes are in agreement with the Irwing-Williams series (Co < Ni < Cu).