Experiments were carried out to develop methods of diffusion welding of joints between single-crystal molybdenum and polycrystalline molybdenum through a titanium interlayer in the hot isostatic pressing (HIP) conditions. The structure and the mechanical and physical properties of the diffusion welded joints Mosingle + Ti + Mopoly were investigated. Composite molybdenum single-crystal mirrors with areas of 5000 and 8000 mm2 were produced and subjected to successful tests.