We investigated photoinduced electron accumulation in a colloidal system of layered hexaniobate that is known as a photocatalytically active wide band-gap semiconductor, and attempted to control the photoresponse by introducing additives into the colloid. The inorganic nanosheets were obtained by exfoliation of the layered oxide. UV-irradiation of the colloids led to electron accumulation in the nanosheets to generate reduced niobate species. Propylammonium ions introduced as the exfoliating reagent and present as the counter ions of niobate nanosheets were indicated as the electron donor that stabilized the electron-accumulating state. Yield and half-life of the reduced niobate species greatly increased by adding an appropriate amount of photochemically inert clay nanosheets, while they increased only a little by the addition of molecular electron donors such as EDTA and triethanolamine. Moreover, the molecular species diminished the enhancement effect of the clay nanosheets. The results suggested that the photochemical event was not explained by direct interactions between the semiconductor nanosheets and the additives at molecular level but governed by indirect interactions between the colloid components regulated by the colloid structure.