Titration potentiometry and spectrophotometry have both been used to determine stability constants for mixed complexes of Cu(II) with creatinine and EDTA. Stability constants of the ternary system were evaluated by a method suggested by the Irving-Rossotti procedure. Conditional constants were established as a function of pH; the maximum values of the conditional formation constants were found to be in agreement with the mixed-ligand complex formation constants in a given pH region. The mole fractions of mixed complexes were calculated using the formation constants. The stability ;constants of mixed-ligand complexes at 25 degrees C are: log K-1 = 5.29, log K-2 = 4.97, with ionic strength being kept constant at I = 0.1 with NaClO4. Also, Cu(II)-I-glutamic acid-creatinine and Cu(II)-EDTA-creatinine complex formation, as examined by the spectrophotometric method, gave results that were in agreement with those of the potentiometric method.