The title compound, {(C(14)H(16)P)[Cu(5)I(6)]}(n), prepared from the reaction between copper powder, iodine and dimethyldiphenylphosphonium iodide in hydroxyacetone, features an anion that consists of a continuous two-dimensional Cu-I sheet [Cu-I = 2.5960 (14)-2.6994 (13) A and Cu-I-Cu = 63.28 (5)-114.25 (5) degrees]. The cation, which lies on a mirror plane, is a typical dimethyldiphenylphosphonium ion. The structure shows a strong tendency towards segregation of the inorganic and organic parts of the structure into separate subspaces. The two-dimensional Cu-I sheet displays a pronounced subcell with pseudo-tetragonal symmetry that is broken by ordered vacancies on the Cu position. The symmetry is further reduced by the orientation of the interleaved organic counter-ion that is inclined with respect to the pseudo-mirror planes defined by the Cu-I sheet normal, perpendicular to the b axis.