A reaction cycle is shown for octahedral tungsten iodide compounds. The thermal transformation of W6I16 (W6I12·2I2) via release of iodine proceeds via the new W6I13 (W6I12·xI2 with 0 < x ≤ 1/2) to yield a new modification of W6I12, denoted as α-W6I12. When heated, α-W6I12 is converted into the known (β-)W6I12. The reaction of (β-)W6I12 with I2 leads to the formation of the starting compound W6I16. The new compounds W6I13 (W6I12·xI2 with 0 < x ≤ 1/2) and α-W6I12 are structurally characterized by powder and single-crystal X-ray diffraction techniques. The thermal decomposition of W6I16 and the monotropic phase transition of α-W6I12 into β-W6I12 are examined by differential scanning calorimetry measurements.