Abstract: The structures of 2 alkylformyl indoles synthetic cannabinoids, ADB-4en-PINACA and MDMB-4en-PINACA, were distinguished by infrared spectrometry and gas chromatography-mass spectrometry, and the 2 compounds in actual tobacco samples were identified. It was shown that the characteristic peaks of ADB-4en-PINACA were located at 3 354.1, 3 208.0, 682.3 cm⁻¹, with retention time of 19.098 min, parent ion of m/z 342.2, and main daughter ions of m/z 213.1, 241.2. The characteristic peaks of MDMB-4en-PINACA were located at 3 411.8, 1 725.4 cm⁻¹, with retention time of 13.238 min, parent ion of m/z 357.2, and main daughter ions of m/z 213.1, 270.1. The reason for the difference between the 2 compounds was that the terminal group of ADB-4en-PINACA was an amide group, while the terminal group of MDMB-4en-PINACA was an ester group. Gas chromatography-mass spectrometry was applied to analysis of samples suspected to contain ADB-4en-PINACA and MDMB-4en-PINACA, and mass spectrometry characteristic peaks obtained were basically consistent with those of the standards.