The thermal expansion of francisite, [Cu_3-BiO₂](SeO₃)₂Cl, a rare copper(II)-bismuth(III) oxide chloride selenite, was studied by high-temperature X-ray powder diffraction over the temperature range 293-773 K. The mineral is stable up to 748 K at which temperature it decomposes to [Bi₂O₂]Se, CuO and, probably, [BiO][CuSe]. The thermal expansion has an anisotropic character (α(a) = 9.0, α(b) = 4.7, α(c) = 17.0 · 10^-6 K^-1), which is determined by the distribution of the bonds between additional oxygen atoms ((a)O) and metal atoms (A). These bonds form two-dimensional systems in which each (a)O atom is tetrahedrally coordinated by one Bi and three Cu atoms. Thus the structure can be described in terms of oxocentered metal atom tetrahedra consisting of [(a)O₂Cu₃Bi] layers of ((a)OCu₃Bi) tetrahedra, (SeO₃) groups and Cl anions. This description gives the most reasonable viewpoint to explain the anisotropic thermal expansion character of francisite.