Size-quantized CdS fine particles with different dimensions were successfully incorporated into Langmuir-Blodgett films. On those samples in-situ observation of high-energy ion-induced emissions and photoluminescence measurements after the irradiation were carried out. The spectra showed various emission bands (centered around 440 nm, 510 nm, 590 nm, 640 nm, and 800 nm) due presumably to different defect states in the bandgap and excitonic emission bands, in agreement with the estimated bandgap energy. Close inspection of those data reveals that : (i) the high-energy ion-irradiation decreases the density of carrier-trapping states in the midgap, which mainly exists in the surface region, and increases the quantum efficiency of the excitonic emission : and (ii) post-irradiation oxidation leads to the stabilization of the S-related defect state and the formation of Cd atoms and results in the generation of the sulfur vacancies.