Ferroelastic domains in γ-KCoPO4 were studied by polarized light microscopy and by x-ray methods. These domains appear upon cooling as a result of a phase transition at ∼449° from an orthorhombic normal (N) phase (with probable space group Pnma) to a incommensurate phase (IC) which is monoclinic and has a av. structure isotypic with CsLiSO4 with space group P21/c. The modulation of the monoclinic structure decreases an the cell parameters change when the crystals are ground to a grain size of <7 μm and heated and cooled several times through the phase transition. A Rietveld refinement of the structure of γ-KCoPO4 was achieved on a powder pattern. The cell parameters are: a 5.3052(3), b 8.5571(4), c 10.3926(6) Å, β 120.833(3)°, Z = 4.