The Raman scattering from Lu1.8Y0.2SiO5 single crystals was investigated for quasi-hydrostatic pressures ranging from atmospheric to about 15 GPa at room temperature. The material, originally belonging to the monoclinic C2/c structure, transforms gradually to a higher-density phase upon the application of pressure. Unit cell volume, peak position, and the Gruneisen parameter have been calculated and analyzed, revealing three different pressure regimes. The first low-pressure regime relates mainly to the change of unit cell volume. In the second pressure regime, the gradual phase transformation starts at about 3 GPa and is completed at a pressure of 10 GPa. The third pressure regime part reveals a sensibly lower pressure dependence. A detailed analysis of the data allowed us to propose a scheme for a pressure-induced gradual phase transition, resulting in the coexistence of phases over a wide pressure range. Moreover, the material reverts to the ambient-condition phase upon release of pressure, showing an almost total reversibility. By comparison with the vibrational spectra of the pure Y2SiO5 compound and from the analysis of the crystal structures, the pressure-induced high-density phase is assigned to the P2(1)/c monoclinic structure, typical of oxyorthosilicates. Copyright (c) 2008 John Wiley & Sons, Ltd.