The crystallisation and crystal structures of three new crystal forms of MePh₃P⁺I₃⁻ are described, bringing to four the number of polymorphs of this compound. Crystallisations were from alcohols, mixed alcohols, and dichloromethane, and are reproducible. There are strong similarities between all four of the crystal packing arrangements, which are solvent-free. In all structures the MePh₃P⁺ cations associate in two-dimensional nets using edge-to-face, offset-face-to-face, and methyl-to-face motifs, and the I₃⁻ ions lie between these nets, with subtle variations in structure caused by sliding displacements of relatively constant cation-anion assemblies. The related compound EtPh₃P⁺I₃⁻ has very similar crystal packing. The interactions between the I₃⁻ and MePh₃P⁺ are mainly C–H⋯I, with some phenyl faces parallel to I₃⁻. The major electrostatic component of the lattice energy is associated with the occurrence of I₃⁻ ions against the faces of nets of cations, consistent with this structural feature being the most invariant aspect of the crystal packing, while the structural variability is associated with the less stabilising cation⋯cation motifs. It appears that these crystals provide a good example of a system where shape is not a dominant feature in crystal packing, and that the tetramorphs represent almost equi-ergic local minima in a relatively deep section of the lattice energy hypersurface. The crystal packing of (MePh₃P⁺)₂I₈²⁻ is similar.