The local microstructure of 25-layer Langmuir-Blodgett (LB) film of cobalt(II) stearate (CoSt(2)) has been studied by small-angle X-ray diffraction, Fourier transform infrared transmission (FTIR-T) and reflection-absorption (FTIR-RA) spectra. The hydrocarbon chains in the CoSt(2) LB film are packed in a hexagonal fashion with all-trans (zigzag) conformation, and the chain axes are aligned normal to the film plane, the long spacing being 50.0 Angstrom. After the reaction of the CoSt(2) LB film with H2S, FTIR and X-ray diffraction measurements showed that two-dimensional CoS monolayers are formed in the LB matrix with Co2+ substituted by H+. Remarkable changes in the structure of the LB film were observed after the insertion of CoS monolayers. The hydrocarbon chains change into orthorhombic subcell packing and tilt on the substrate, and the long spacing becomes 40.0 Angstrom.