In this paper, we study the effect of Ni(II) doping on the structure of L-histidine hydrochloride monohydrate crystals using x-ray diffraction and Raman spectroscopy. X-ray powder diffraction shows no significant change in the unit cell parameters of the doped single crystal, whereas x-ray multiple diffraction using synchrotron radiation indicates that the Ni ions are located in interstitial positions in the crystal lattice. The temperature-dependent Raman spectra reveal a structural phase transition in the 10-300 K temperature range. The proposed mechanism of this phase transition supports the suggestion that the Ni ions occupy interstitial positions.2027