Abstract The melting point-composition diagram for the thallium-iodine system has been determined. There are three compounds: TlI, Tl 3I 4, and TlI 3. From Tl to TlI the solids and liquids are immiscible. From TlI to I 2 there is one liquid phase. Solid Tl 3I 4 melts incongruently at 260 °C to a liquid (atom fraction iodine = 0.77) and solid TlI. Solid TlI 3 melts incongruently at 129 °C to a liquid (atom fraction iodine = 0.84) and solid Tl 3I 4. A eutectic occurs with liquid (atom fraction iodine = 0.88), solid TlI 3, and solid I 2 at 90 °C. The X-ray powder patterns of Tl 3I 4 and TlI 3 are reported.