Raman spectra of an L-methionine (C5H11NO2S) crystal were obtained in the spectral region between 50 and 3200 cm−1 for pressures up to 5 GPa. Pronounced changes of the Raman spectra were observed for bands associated to rocking of CO2−; wagging of CO2−; deformations of CO2−, CH3, and NH3+; and stretching vibrations of SC, CC, CH, CH2, and CH3. Upon decompression to ambient pressure the original Raman spectrum prior to compression is recovered. These modifications were associated to a reversible phase transition undergone by the L-methionine crystal at about 2.2 GPa, with a hysteresis of ∼0.8 GPa. Pressure coefficients for most of the internal modes of the crystal are given. Copyright © 2008 John Wiley & Sons, Ltd.