LaHo 0.75 Sr 0.25 CuO 3.9 , representing the intergrowth of rock salt, fluorite-type slabs, and (CuO 2 )-layers have been fluorinated using XeF 2 as a soft fluorinating agent at temperatures ranging from 200 to 400°C. The resulting structures were studied by a combination of X-ray diffraction, electron diffraction, and high resolution electron microscopy. The samples show a complex microstructure with alternating bands of two fluorinated phases with different structures. Narrow bands (60–100 Å) of a monoclinic phase (a m =a√2≈5.5 Å, b m =a√2≈5.5 Å, c m =c=12.84 Å, β=91.4°) are formed in an orthorhombic matrix. The formation of the monoclinic superstructure (A phase) is accompanied by an equal anion arrangement within the initially different rock salt and fluorite-type slabs and by cooperative displacements of the A cations. In the structure of the less fluorinated B phase, the fluorite slabs are not affected by fluorination. Fluorine atoms occupy part of the interstitial positions in the rock salt slabs with partial removal of apical oxygen atoms; this lies most probably at the origin of the incommensurate modulation of the B phase.