The formation and growth of Q-state CdS particles during exposure of Langmuir-Blodgett films of cadmium arachidate (CdAr) to hydrogen sulfide have been studied using grazing angle Fourier transform infrared (FTIR) spectroscopy. Films were studied after prolonged exposure to H2S (1 h) and after being subjected to repeated cycles of H2S exposure (1 h) and subsequent immersion in an aqueous solution containing cadmium chloride (1.5 h). During each H2S exposure, the CdAr molecules in the film were converted to arachidic acid (ArH). A small proportion of the film, however, was present in the form of the cadmium salt after prolonged exposure to H2S. Immersion of the film in CdCl2 solution resulted in reconversion of ArH molecules into their cadmium salt. An increase in the average angle of tilt of the surfactant molecules with respect to the gold substrate occurred in conjunction with Q-state particle formation. The degree of tilt increased with increasing numbers of gassing and immersion treatments. Processes such as the conversion of CdAr to ArH as well as breakup of the film into microdomains appear to be responsible for an increase in tilt of the alkyl chains. The ionized arachidic acid molecules which were found to be present in the films after prolonged exposure to H2S appear to form a capping layer around the CdS particles. The presence of a layer of ionized arachidic acid molecules around the particles may also be responsible for an increase in the average angle of tilt of the alkyl chains.