A quantitative chromatographic separation followed by UV spectrophotometric determination is presented for the analysis of a commercial tablet formulation containing mephobarbital and diphenylhydantoin. A patented activated magnesium silicate is used for the chromatographic column separation. Mephobarbital is eluted with 4% ethyl acetate in chloroform and diphenylhydantoin is eluted with 10% methanol in chloroform.