Because of the occurrence in the illicit drug market of clandestinely synthesized analogs and homologs of therapeutically useful drugs (often referred to as "designer drugs"), there is need for improved methods for differentiating these scheduled compounds from unregulated substances. In addition, clandestinely prepared drugs often contain impurities of manufacture that are structurally similar to those of the desired drug. This study analyzed a series of ring-substituted (4-methyl, 4-methoxy, and 3,4-methylenedioxy) amphetamine and N-methylphenethylamine isomers using color tests, thin-layer chromatography, gas chromatography/mass spectrometry (GC/MS), and GC/infrared (GC/IR). The N-acetyl derivatives of the isomers were analyzed using GC/IR/MS. GC/IR/MS readily differentiated the 4-methylphenylalkylamine isomers and the 3, 4-methylenedioxphenylalkylamine isomers, but differentiation via GC/IR/MS was difficult. The N-acetyl derivatives of each pair of isomers could be readily differentiated using GC/IR/MS. Good library researchable spectra for N-acetylamphetamine could be obtained for IR identification with 10 ng (on-column) and MS identification with 2 ng. The spectrometrically independent IR and MS data obtained for the N-acetyl derivatives indicated that the combination of GC/IR/MS can add a significant level of confidence in the analysis or ring-substituted arylalkylamines. 7 figures, 3 tables, and 27 references (Author abstract modified)