Phenobarbitone has been determined in a ternary mixture with oxyphenonium bromide and meprobamate (mixture A) and in binary mixtures with paracetamol (mixture B) or acetylsalicylic acid (mixture C) using derivative spectrophotometry applying zero-crossing technique of measurement. The combining active ingredients have also been determined. The procedures were proved using laboratory prepared mixtures and their utility in assay of pharmaceutical dosage forms were also presented.