The photoelectrochemical (PEC) characteristics of thin films of ‘Q-state’ CdS semiconductor particles in arachidic acid Langmuir–Blodgett (LB) layers have been examined. Initially, ‘Q-state’ size CdS particles of ca. 2 nm were formed in the film and then progressively grown to ca. 10 nm by a cyclic sequence, which involves exposing the film to H₂S(g) following its immersion in a CdCl₂(aq) solution. A PEC cell was constructed using a layered glass/ITO–CdS/LB plate as the photoanode and a standard calomel electrode (SCE) as the reference electrode. The photoelectrical response of electrodes coated with LB films containing CdS particles of various sizes was investigated using white light. The reproducibility of current–voltage polarisation curves indicated the relatively good stability of the film to PEC degradation processes over several scanning cycles in the range 0 to –1000 mV vs. SCE. Values for the open-circuit (V_oc) and the short-circuit current for the PEC cells were obtained and fill factors in the range 20 to 70–75% were calculated. V_oc decreased from ca. –200 to ca. –800 mV vs. SCE with increasing particle size. This effect was attributed to the dielectric properties of the arachidic acid LB film matrix.