A derivative spectrophotometric method has been developed for quantitative determination of p-coumaric and ferulic acid6 in mixtures. A detailed study of experimental variables and instrumental parameters was carried out. This paper reports the testing of a graphical model to measure derivative amplitudes which is based on the interference-free character of the isodifferential points in the derivative calibration graphs. Detection limits of 20 and 50 ng/ml and r.s.d.'s of 0, 63 % and 1, 73 % were found for p-coumaric and ferulic acids, respectively.