Abstract: The zero--order derivative absorption spectra of aspirin, paracetamol and caffeineare closely overlapping. In this paper, a zero--crosssing derivative spectrophotometry combined withsimultaneous equations is proposed for the determination of the three components in a mixture withoutthe need of prior separation. Satisfactory results were obtained. The method is reproducible, accurateand rapid. Calculation is not complicated. This method could be applied to biological fluid analysis.