Casting a photopolymer solution to form a film, followed by image-wise photoirradiation and subsequent wet development, leads to photolithography. Whereas the wet development is achievable with aqueous alkali, the emission of an organic solvent as a volatile organic compound (VOC) is usually inevitable during the film casting because ingredients of common photopolymers are insoluble in water. This work proposes water-borne photopolymers cast and developed solely with neutral water by incorporating mechanically milled sub-micron particles of poorly water-soluble photoacid generators (PAGs), which undergo direct and sensitised solid-state photolysis to make a poly(vinyl alcohol) film insoluble in water with the aid of acid-sensitive crosslinkers. The sensitised solid-state photolysis of onium-type PAGs in the solid state was studied in detail to extend the spectral sensitivity of this kind of photopolymer. Fluorescence quenching measurements revealed that the solid-state sensitisation occurs through singlet exciton migration in sensitiser particles followed by electron transfer to PAG particles at the solid–solid boundary region. Three-component photopolymers comprised of an aqueous dispersion of PAG in the absence or in the presence of a co-milled sensitiser, poly(vinyl alcohol) and a water-soluble diepoxy were formulated to display photosensitivity of 17 mJ cm⁻² by the direct photolysis of a dispersed sulfonium salt and that of ca. 120 mJ cm⁻², for instance, by the photolysis of an iodonium salt sensitised with co-milled 9,10-dipropoxyanthracene.