tert-Butyl(isopropyl)iodophosphane (1) reacts with 1 equiv of iodine in dichloromethane furnishing yellow tertbutyl(isopropyl)diiodophosphonium iodide t-Bu(i-Pr)PI3 (2). A byproduct is red crystalline t-BU(i-Pr)PI3.25 (3) Stepwise addition of iodine to 1 provides 2 and the tert-butyl(isopropyl)diiodophosphonium iodide/triiodides t-Bu(i-Pr)PL (4) and t-BU(i-PT)PI4.33 (5) NMR spectra from solutions prepared by adding various amounts of iodine to 1 always show only one P-31-NMR signal and one set of H-1-NMR resonances; chemical shifts and coupling constants, which depend on the amount of iodine added to 1, indicate rapid exchange equilibria in solution and decreasing cation-anion interactions with increasing iodine content. The solid state structures of 2-5 were determined by X-ray crystallography. 2 (orthorhombic, space group Pbcn, Z = 8, a = 13.413(3) Angstrom, b = 14.693(3) Angstrom, c = 14.240(3) Angstrom) consists of helical chains of tert-butyl(isopropyl)diiodophosphonium ions bridged by mu(2)-iodide anions through iodine-iodine interactions. t-Bu(i-Pr)PI3.25 (3) (tetragonal, space group P4/ncc, Z = 16, a = 19.730(3) Angstrom, c = 14.485(4) Angstrom) is [t-BU(i-Pr)PI2](4)(mu(2)-I)(2)(mu(4)-I)[I(I-2)(x)] (mixed-Valence columnar structure, x approximate to 0.5) : 16-membered rings from four t-Bu(i-Pr)PI2-I moieties are connected in a spiro fashion by mu(4)-iodide anions; each channel between four of these cationic stacks is occupied by iodine ([I(I-2)(x)](-), disordered). The backbone of the chain structure of crystalline 4 (orthorhombic, space group P2(1)2(1)2(1), Z = 4, a = 9.462(2) Angstrom, b = 11.130(3) Angstrom, c = 30.228(6) Angstrom) is formed by [-P-I-I-I-] units from tert-butyl(isopropyl)diiodophosphonium ions and mu(3)-donor iodide ions, each of which is connected with one iodine atom of a further side chain tertbutyl(isopropyl)diiodophosphonium ion. The other iodine atom of this side chain cation is coordinated by a terminal triiodide ion. 5 (monoclinic, space group P2(1)/c, Z = 4, a = 23.874(9) Angstrom, b = 21.858(7) Angstrom, c = 9.477(3) Angstrom, beta = 94.30(3)degrees) contains a double chain backbone of mu(2)-acceptor phosphonium ions and mu(5)-bridging donor iodide ions. Four I ... I contacts of these mu(5)-bridging donor iodide ions are within the double chain, and the fifth is to a side chain tert-butyl(isopropyl)diiodophosphonium ion, which is also connected to a terminal triiodide anion by an I I interaction. A further triiodide anion within the unit cell has no soft-soft interaction with the cationic network.