A combined structural refinement of Bi(3.5)Lao(0.5)Ti(3)O(12) against both neutron and X-ray diffraction data was performed at 298 K on the basis of the Raman study. The upshift of Raman peaks suggested that the substitution sites of La atoms in Bi3.5La0.5Ti3O12 were only the Bi sites in the perovskite units. Of the two crystal structural models (orthorhombic and monoclinic systems) considered for the crystal structural system of Bi3.5La0.5Ti3O12, the weighted R factor, R-wp, and goodness-of-fit indicator, S (=R-wp/R-e), of the monoclinic system were lower than those of the orthorhombic one. The final R,p and S values based on the monoclinic system were 7.04% (6.34 and 7.76% for the neutron data and the X-ray data, respectively) and 1.45, respectively. The lattice parameters obtained from the combined structural refinement were a = 5.4321(1) Angstrom, b = 5.4161(1) Angstrom, and c = 32.8614(3) Angstrom. The beta angle was 89.95(4)degrees. Spontaneous polarizations calculated from the refined structural parameters were 27.0 muC/cm(2) for the monoclinic system and 1.8 muC/cm(2) for the orthorhombic one.