A derivative spectrophotometric method was developed for quantitative determination of ternary synthetic organic dyes tartrazine (E102;T), indigo carmine (E132, IC) and erythrosine (E127; E), which are under governmental regulations all over the world because of their toxicity and carcinogenity. The main obstacle in the spectrophotometric determination of mixtures of these colourants is the overlapping of their spectra. Since the three colourants studies here present spectral overlapping their simultaneous determination is different when conventional method are used. In this study ternary mixtures of colourants tartrazine, indigo carmine, erythrosine are resolved by using the first and second derivative spectrazero crossing method without the need for any seperation step. The amplitudes of the first derivative at 453 and 639 rim were used for determination of tartraiine and indigo carmine in the concentration range of 0.5-2.5 and 10.0-80.0 mu g/mL, respectively. The amplitude of the second derivative at 561 nm was used for determination of erythrosine in the concentration range 0.2-1.0 mu g/mL. This method was successfully applied to commercial pharmaceutical capsule sample containing the three colourants and results were compared with HPLC method.