Phase equilibria in the Bi 2 TeO 5 Bi 2 SeO 5 system were studied by X-ray, DTA and second harmonic generation (SHG). The samples were synthesized by solid state reactions of the Bi, Te and Se oxides. The phase diagram is interpreted as a quasibinary peritectic one with wide ranges of solid solutions on the basis of both compounds. The SHG study showed Bi 2 SeO 5 to undergo a phase transition at about 250 o C. Neutron diffraction (25-650 o C) showed no major changes in the structure of Bi 2 SeO 5 at high temperatures. However, the analysis of the oxygen atom thermal factors and site occupancies suggested that the mechanism of the phase transformation is an order-disorder transition involving reorientation of the SeO 3 group.