The formation of Q-state CdS particles in cadmium arachidate (CdAr) Langmuir-Blodgett films has been monitored using the quartz crystal microbalance (QCM) and UV-vis absorption measurements. QCM studies showed that the mass uptake of different thickness CdAr films on exposure to hydrogen sulfide was consistent with that expected for the quantitative conversion of Cd2+ ions in the films to CdS and the corresponding conversion of CdAr to arachidic acid. UV-vis absorbance measurements showed an optical blue shift typical of particles between 2 and 3 nm in diameter. Complimentary electron microscopic studies showed a polydisperse size distribution of CdS particles. The most common particle size observed in the electron micrographs was found to be close to the particle size determined from the optical properties of the semiconductor particles. There was a linear relationship between the mass uptake of a film on H2S exposure and changes in its absorption properties due to Q-state particle formation. The linear relationship was used to calculate the optical extinction coefficient of the Q-state particles in the film. Mass uptake measurements on CdAr films extensively exposed to H2S gas showed that the arachidic acid molecules left after Q-state particle formation could be quantitatively converted to CdAr by immersing the film in a stirred aqueous solution containing cadmium chloride. Subsequent reexposure of the film to H2S increased the mole fraction of CdS in the film.