The low temperature (LT) modification of Cs3Bi2I9 has been refined to R(F) = 0.055 on the basis of X-ray diffraction measurements at T = 130 K from a twin crystal with six fractions connected by 3-fold axis and m-planes symmetry operations. Data were collected on a KM4CCD diffractometer; MoKalpha-radiation; 1769 independent reflections with F > 4 sigma (F) were obtained. Below the LT phase transition each reflection were split. The absorption correction was accounted for by the Gaussian integration method. The parameters of the monoclinic unit cell are: alpha = 8.346(8), b = 14.47(1) and c = 21.10(1) Angstrom, beta = 91.00(7)degrees; space group C2/c. The structure of the monoclinic LT phase is slightly distorted in comparison to the hexagonal RT phase (a(m) = a(h); b(m) = 2a(h) + b(h), c(m) = -c(h)). The phase transition at T = 218 K between the RT acid LT modifications is reversible. The additional maxima observed in the Fourier-syntheses are associated with coherent parts in domains belonging to various twin fractions. The nature, the probable structure and the temperature variations of the domain boundaries are considered and discussed. It is suggested that the specific effect observed on I-127 NQR spectra for the LT modification between 205 < T K < 218 may be due to the incommensurate nature of the domain boundaries. [References: 7]