We developed a method for determining the following ten psychedelic phenethylamines and tryptamines by gas chromatography-mass spectrometry (GC-MS). The phenethylamines examined were 3,4,5-trimethoxyamphetamine (TMA), 2,4,5-trimethoxyamphetamine (TMA-2), 4-bromo-2,5-dimethoxyphenethylamine (2C-B), 4-iodo-2,5-dimethoxyphenethylamine (2C-I), 2,5-dimethoxy-4-ethylthiophenethylamine (2CT-2), and 2,5-dimethoxy-4-propylthiophenethylamine (2CT-7). The tryptamines examined were 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT), α-methyltryptamine (AMT), N-isopropyl-5-methoxy-N-methyltryptamine (5-MeO-MIPT), and N,N-diisopropyl-5-methoxytryptamine (5-MeO-DIPT). As the reference standards, five compounds were newly synthesized, and five were purified from the products. These were all structurally ascertained by GC-MS and nuclear magnetic resonance (NMR) spectroscopy. Between April 2005 and March 2007, 8 compounds analyzed in this study were found in 100 out of the 178 products which were examined.