The Bi2Te3-Bi2Se3 and Bi2Se3-In2Se3 systems were studied by differential thermal analysis and X-ray powder diffraction. The phase diagrams of these systems were constructed; they are quasi-binary. In both cases, there are wide ranges of solid solutions based on the terminal compounds and an intermediate range of demixing. The solid solutions are of tetradymite type (rhombohedral R(3) over barm$). Their limits are related to the substitution of the selenium-tellurium and bismuth-indium atoms in the layers. Electrical conductivity and thermoelectric power measurements confirm the boundaries of the solid solution regions deduced from the thermal and X-ray diffraction data.