CdS Q-state particles, with mean diameters varying from 2 to 10 nm, grown in arachidic acid Langmuir–Blodgett (LB) films, have been exposed to H₂Se(g) to form the corresponding Q-state CdS_xSe_{(1 –x)} particles. These particles are considered to be made up of a core of CdS coated with about a monolayer of CdSe. Q-state CdS_xSe_{(1 –x)} particle formation was verified by X-ray photoelectron spectroscopy (XPS) and by monitoring a red shift in the UV–VIS absorbance spectra relative to that of CdS. XPS results on 6 nm diameter CdS particles that had been grown in an LB film and then extensively exposed to H₂Se(g), revealed a stable average composition of CdS_0.4Se_0.6. A study of the photoelectrochemical behaviour of these systems was conducted through current–voltage polarisation curves in the range 0 to –1000 mV vs. SCE. An average drop of 100 mV in the open-circuit voltage and a marked increase in the short-circuit current was observed when LB films with Q-state CdS particles were exposed to H₂Se(g). Evidence is also presented which shows that, as the mean diameter of the core-shell particles increases from 2 to 10 nm, better connection between the particles in the film is achieved.