Reactions of dialkyldiiodophosphanes R(2)PI, 1 (1a, R = t-Bu; 1b, R = i-Pr; 1c, R = Et) with various molar ratios of iodine were followed by NMR spectroscopy in solution and by X-ray crystal structure determinations of solid compounds R(2)PI(n) (n = 3, 4, 5). In solution at room temperature, rapid iodine transfer reactions occur between P-V-compounds R(2)PI(3) (2a, R = t-Bu; 2b, R = i-Pr; 2c, R = Et) and the iodophosphanes. With increasing iodine content of R(2)PI(3)/I-2 systems, decreasing I ... I interactions between cations (R(2)PI(2)(+)) and anions (I- > I-3(-)) are indicated by solution NMR data such as P-31 downfield shifts and increasing (3)J(P-31, H-1). Cation-anion interactions are weakest when I- anions are trapped as AlI4- anions by addition of aluminum triiodide (5a, t-Bu(2)PI(2)(+) A1I(4)(-)). Structure determinations of t-Bu(2)PI(3) (2a, C8H18I3P, orthorhombic, space group Pmmn, a = 9.208(2) Angstrom, b = 11.482(2) Angstrom, c = 6.970(2) Angstrom, Z = 2; mu(2)-bridging I-), (i-Pr2PI2)(2)(I)(I-3) (3b, C12H28I8P2, monoclinic, space group P2(1)/n, a 11.919(3) Angstrom, b = 10.156(3) Angstrom, c = 25.214(5) Angstrom;alpha 90 degrees, beta = 98.48(2)degrees, gamma = 90 degrees, Z = 4, mu(3)-bridging I-, terminal I-3(-)) and Et(2)PI(5) (4c, C4H10I5P, monoclinic, space group P2(1)/n, a = 8.294(4) Angstrom, b = 14.516(5) Angstrom, c = 12.315(5) Angstrom, alpha = 90 degrees, beta = 91.73(3)degrees, gamma = 90 degrees, Z = 4, bridging I-3(-)) reveal that within the chain structures of 2a, 3b and 4c, R(2)PI(2)(+)....I- interactions are significantly stronger than R(2)PI(2)+... I-3(-) interactions. In each case, both iodine atoms of the R(2)PI(2)(+) cations act as soft accepters via iodine toward soft I- and/or I-3(-) anions.