Physical characteristics are reported for a tablet form of a new hallucinogenic drug previously circulating in the USA under the name “STP”. High resolution mass, nmr, ultraviolet and infrared spectrometric evidence, which identify the extracted base as 2,5-dimethoxy-4-methylamphetamine, and its chromatographic behaviour, are compared with the experimental compound “DOM”. Polymorphic modifications exhibiting distinct solid phase infrared spectra have been studied by X-ray diffraction and by differential calorimetry. Animal behavioural tests indicate that the psychotomimetic activity of the base is comparable with mescaline but up to 50 times more potent.