Abstract The displacive ferroelectric Bi4Ti3Ol2 [Mr= 1171.6, a= 5-450 (1), b= 5.4059 (6), c= 32-832 (3)~, r= 90" 00, Z= 4, Dx= 8" 045gcm-3, MoKa, A= 0-7107, A,,/z= 747.3 cm-i, F (000)= 1976] is described at room temperature as a commensurate modulation of an Fmmm parent structure derived from an idealized 14/mmm structure. Displacive modes of inherent F2mm, Bmab and Bbab symmetry are all substantial and reduce the space-group sym-metry to B2ab. A further substantial displacive mode of Bbam symmetry reduces the space-group symme-try to Blal and induces minor displacive modes of Fmm2, F12/ml and Bream symmetry. A group-theoretical analysis of the problem details how the X-ray data can be classified so as to monitor the refinement. To a first-order approximation, the F2mm and Bbab symmetry components of atom displacements contribute only to the imaginary part of the structure factors. Because the structure factors have a predominant real component, it is easy to get the F2mm and Bbab displacive components of the O atoms wrong. False minima occur at values of 0.027 for R~= Y. hllFobs (h) l-IFcalc (h) ll/ZhlFobs (h) l but a corrrect solution was obtained with RI= 0.0177 for 2839 unmerged data with l (h)> 2o-[I (h)]. Criteria for a correct solution are evaluated and subsequently met. The final refinement used a 0.63: 0-37 twin model.