In this work we investigate the structural properties of Ce:Lu2(1−x)Y2xSiO5 (LYSO) crystals as a function of cerium incorporation by means of low-temperature high-resolution Raman measurements. Czochralski grown single-crystal LYSO samples were obtained with a ratio of yttrium to lutetium ions 10:90 (Ce:Lu1.8Y0.2SiO5) and a variation in the cerium concentration in the 0–750 ppm range. A red shift of the Raman bands, related to a decrease of the unit cell, is observed as function of the cerium concentration in the crystal. By analyzing the low wave number vibrational modes and the polarized Raman measurements, the substitutional role of cerium ions in the rare-earth sites occupied by yttrium ions is infrared. In addition, since the rare earth can occupy two independent different crystallographic sites, the analysis of the Raman spectra allows to distinguish the preferred site by cerium ions in the doping process.