An amphiphilic oligomer, polymaleic acid with octadecanol ester (PMAO), with ratios of 2:1, 3.7:1 and 6:1 between the carboxylic groups and the hydrocarbon chains, was synthesized and has been used as a matrix to form and grow quantum dot-sized CdS particles. The formation of quantum-sized CdS particles during exposure of Langmuir-Blodgett (LB) films of CdPMAO to hydrogen sulfide has been studied using Fourier transform infrared spectroscopy and UV-vis absorption spectra. This showed that hydrogen bonding of the PMAO molecules were replaced by the metal ion (Cd2+) bridges to form the bridging complex structures before the reaction with H2S. The intensities of the asymmetric nu(a)(COO-) and symmetric nu(s)(COO-) stretching modes were reduced after the reaction. But the presence of the asymmetric nu(a)(COO-) and symmetric nu(s)(COO-) stretching mode in spectra implied that not all the CdPMAO oligomers in the film were converted to PMAO after exposure to H2S gas. The result from the UV-vis absorption spectra indicated that the CdS within PMAO LB films showed a larger blue shift of the optical absorption edge with the decrease of the ratio between the carboxylic groups and the hydrocarbon chains. Moreover, it could be deduced that the size of the aggregate of the sulfides within PMAO LB films was smaller than that formed in stearic acid LB films.