Triphenylmethylphosphonium triiodide [P(C6H5)3CH3]I3 has been prepared by the reaction of [P(C6H5)3CH3]I with iodine in ethanol. Deep violet crystals were obtained by recrystallization from dichloromethane. The crystal and molecular structure of the compound has been determined. The crystals are orthorhombic, space group Pbca, Z = 8, a = 1182.2(2), b = 1987.6(3), c = 1798.8(3) pm. Least squares refinement of the structure yielded R = 0.038 (Rw = 0.030, RG = 0.030) for 2433 independent reflections. The triodide ion I3- is slightly asymmetrical and slightly bent. The distances are 293.7(1) pm (I(1)—I(2)) and 291.7(1) pm (I(2)—I(3)), the angle I(1)—I(2)—I(3) is 179.3(1)°. The structure contains layers perpendicular to the c axis, which follow each other with a period of c/2 and are built up by I3- anions and [P(C6H5)3CH3]+ cations. They are stacked to give layers of cations and layers of anions perpendicular to the b axis. The shortest I ··· I distance between I3- ions in the anion layer occurs in the c direction. It amounts to 414 pm, thus indicating that there are only very weak I ··· I interactions between the I3- ions. Further I ··· I contacts in the layer lie between 510 and 746 pm.