The preparation of a visible light-absorbing, very thin (2.5 m), flexible CdS nanoparticle coated polystyrene (PS) film, CdS-PS, with a 3D-printed backing is described. Scanning electron microscopy (SEM) confirms that the CdS-PS film comprise a thin layer of CdS nanoparticles (26 4 nm) on just one side of the PS film, with no nanoparticles on the other side. When irradiated with 420 nm or 365 nm radiation, in air-saturated water, the CdS film is photobleached, and dissolved O2 consumed, due to the photoinduced oxidative corrosion of the CdS by O2. In contrast, under the same aerobic conditions, the CdS-PS film is very stable, when a sacrificial electron donor (SED) is present, such as EDTA or ascorbate/ascorbic acid, with the latter appearing the most effective. In the presence of an SED, the CdS-PS film photocatalyses the reduction of the dyes, methyl orange and crystal violet, and the electron-relay, methyl viologen, by different SEDs, using visible and UV light. In the photocatalysed reduction of methyl viologen by EDTA, colloidal Pt reacts with the highly coloured blue methyl viologen radicals generated to produce H2. Visible light irradiation of the CdS-PS/MV2+/EDTA/colloidal Pt system promotes the reduction of water to H2 by the SED, EDTA, mediated by methyl viologen. A colourless, TiO2-PS film, made using P25 TiO2, is used to effect the same photocatalytic reduction reactions as the CdS-PS film, but only when irradiated with UV (365 nm) radiation. In both cases the films are used repeatedly with no evidence of deterioration in activity or film stability. This is the first example of the preparation and testing of a visible light absorbing photocatalytic, i.e. CdS, thin plastic film, the preparation of which is very simple and inexpensive and may prove invaluable for the production of thin, flexible plastic photocatalytic films for solar research.