Single crystals of the title compound, potassium hexaphosphitopentaferrate(II,III) hemihydrate, K0.75[FeII3.75FeIII1.25(HPO3)6]·0.5H2O, were grown under mild hydrothermal conditions. The crystal structure is isotypic with Li1.43[FeII4.43FeIII0.57(HPO3)6]·1.5H2O and (NH4)2[FeII5(HPO3)6] and exhibits a [FeII3.75FeIII1.25(HPO3)6]0.75−open framework with disordered K+(occupancy 3/4) as counter-cations. The anionic framework is based on (001) sheets of two [FeO6] octahedra (one with point group symmetry 3.. and one with point group symmetry .2.) linked along [001] through [HPO3]2−oxoanions. Each sheet is constructed from 12-membered rings of edge-sharing [FeO6] octahedra, giving rise to channels with a radius ofca3.1 Å where the K+cations and likewise disordered water molecules (occupancy 1/4) are located. O...O contacts between the water molecule and framework O atoms of 2.864 (5) Å indicate hydrogen-bonding interactions of medium strength. The infrared spectrum of the compound shows vibrational bands typical for phosphite and water groups. The Mössbauer spectrum is in accordance with the presence of FeIIand FeIIIions.