The reaction of dpma (dpma = (dimethylphosphoryl) methanamine) with hydroiodic acid in the presence of an excess of elemental iodine gives red coloured crystals of (dpmaH)3(H3O)I4 · ½I2 (1) as the main product and a small amount of a by-product (dpmaH)2[I3]2 · dpma (2). Single crystal structure determinations on both compounds were carried out. In the crystal structure of 1 (space group P3, a = 12.2166(3) Å, c = 10.8788(4) Å, Rgt(F) = 0.0224, wRref(F²) = 0.0377), six symmetry related dpmaH+ cations and iodide anions, both located in general positions, form a cage through NH· · ·I hydrogen bonds. These cages are connected to neighbouring ones by H3O+ cations by means of strong OH· · ·O hydrogen bonds, thereby constructing a 2-dimensional network perpendicular to the c direction. In the center of each cage a three-fold disordered I2 molecule is present. This disorder is a consequence of space group symmetry. Each iodine molecule forms two halogen bonds to two adjacent iodide anions (Δ(I· · ·I) = 3.2449(7) Å) formally resulting in an almost linear I4 2- anion. The second iodide counter anion is located alternatively beneath and below the H3O+ cation and is not involved in any stronger hydrogen bonding interaction. The dominant feature of the crystal structure of 2 (space group C2/c, a = 28.885(5) Å, b = 10.6854(15) Å, c = 20.659(4) Å, β = 110.64(2)°, Rgt(F) = 0.0776, wRref(F²) = 0.1410) is a ladder substructure formed by dpmaH cations and neutral dpma molecules via NH· · ·O and NH· · ·N hydrogen bonds. Triiodide counter anions occupy gaps in the ladder. A characteristic Raman signal representing the iodine molecule in the Raman spectrum of 1 is shifted to lower wavenumbers (173 cm-1). It corresponds to an iodine molecule trapped in the cage of a hydrogen bonded network forming medium strong halogen bonds. The Raman spectrum of 2 shows the typical bands of a weakly connected I3-anion. [ABSTRACT FROM AUTHOR]