The EI mass spectra for these isomers show a number of unique ions which allow for the differentiation of the 1-alkyl-3-acylindole compounds from the inverse regioisomeric 1-acyl-3-alkylindoles. The base peak at m/z 214 in the 1-n-pentyl-3-benzoylindole represents the M-77 cation fragment resulting from the loss of the phenyl group, and this ion is not observed in the inverse isomer. The 1-benzoyl-3-n-pentylindole inverse regioisomer shows a base peak at m/z 105 for the benzoyl cation. Thus, these two base peaks are the result of fragmentation initiated at the carbonyl-oxygen for both isomers. The 1-pentyl-3-benzoylindole is characterized by the strong intensity carbonyl band at 1703 cm-1, and the amide carbonyl appears as a strong band of equal intensity at 1681 cm-1 in the 1-benzoyl-3-pentyl regioisomer. (Publisher abstract modified)