Infrared spectra of LSD and its tartrate salts, and factors affecting their reproducibility, have been investigated. Spectra corresponding to amorphous and crystalline forms of LSD base, the neutral tartrate and two forms of the hydrogen tartrate were obtained. The neutral tartrate was found to undergo conversion to the hydrogen tartrate on long standing. Comparison with spectra of related compounds shows that all the LSD spectra are distinctive and can be used for identification purposes, but the use of potassium bromide discs was found to cause spectral changes in the salts.