An attempt to distinguish between 1:1 and 2: 2 weak complexes from continuous variation data is given. A modification of the Heller and Schwarzenbach's method which involves the use of the absorbance ratio y=A/A(lim) (once the limiting absorbance, A(lim) is known) may be applied to 1:1 weak complexes. This allows checking if the slope obtained is close to the theoretical value of -0.25. A curvature appears and the residuals have a pattern when a wrong 1:1 model is selected for a true 2:2 stoichiometry.