A new spectrophotometric multicomponent analysis method, for resolving a ternary mixture of sulphaquinoxaline (SQX), sulphamethazine (SMT) and pyrimethamine (PMT) on the basis of the simultaneous use of the first derivative of ratio spectra and measurements of zero-crossing wavelengths is described. The procedure does not require any separation step. The optima conditions were established and the method was tested in synthetic mixtures (for SQX:SMT:PMT ratios from 1:1:4 to 1:4:1 and to 4:1:1). Finally, the method is applied for the quantification of the three compounds in a commercial product and the found results are compared with those ones obtained by PLS-2 and HPLC.