Formation of ultrathin films of CdS nanostructures on solid substrate has been achieved by selective removal of the fatty acid matrix from Langmuir-Blodgett films of cadmium arachidate exposed to an atmosphere of hydrogen sulfide. By means of nanogravimetric assays it was possible to monitor the formation of CdS nanoparticles during the reaction and to reveal the presence of a film, nonsoluble in organic solvent, after treating the sample with chloroform. Changes in the film structure due to the formation of CdS particles were shown by X-ray study. Optical ellipsometry made it possible to measure the average thickness of the obtained film. Optical absorption spectra proved the presence of Q-state CdS particles before and after removing the fatty acid matrix and showed, moreover, an increase of the effective size of the particles from about 3 to about 4.4 nm. STM of the CdS residual film revealed, in turn, an interconnected structure formed by bumps of about 2-3 nm in diameter when the film was obtained from one cadmium arachidate bilayer and a similar structure with bumps of increased size (4-5 nm) in the case of a sample derived from 20 bilayers, indicating that a mechanism of particle aggregation is responsible for the semiconductor film formation. In the latter case, atomic resolution was achieved, revealing a CdS lattice.