We succeeded in characterizing by long-wavelength neutron diffraction light-induced volume phase holograms which have been recorded in photopolymers. The samples were prepared from protonated and deuterated poly-(methylmethacrylate). They initially contained about 10 - 15% monomers and a photo-initiator. Discs of about 2 mm thickness and 2 to 3 cm diameter were exposed, in a conventional two-beam interference setup, to a periodic light intensity pattern, with fringe spacings between 120 and 390 nm. The illumination modulated the neutron optical refractive index. This was due to a density variation that was introduced by a diffusion-controlled photopolymerization, within the light regions, of the residual monomers. The neutron diffraction has been observed using the full length of the 80 m small-angle camera D11 at the High-Flux Reactor, Grenoble, France. The primary neutron beam was monochromatized at around (lambda) equals 1 nm within (Delta) (lambda) /(lambda) equals 18% full width and collimated within 0.75 mrad. The amplitude of the neutron refractive index modulation of the protonated samples was by 105 smaller than the light refractive index modulation if using a helium-neon laser beam of (lambda) equals 633 nm. The neutron reflectivity of the fully deuterated samples, 1.4%, was by a factor 40 larger than that of the protonated samples; this makes these gratings valuable etalons for neutron small-angle cameras. Possible further applications of our research include the study of light-induced neutron refractive index changes and photochemical processes in situ using a radiation which does not alter the structure; investigation of the factors that influence the regularity and stability of the holographic gratings; development of neutron optical devices as beam splitters, mirrors, and lenses; and, in the more distant future, the construction of instruments, e.g., neutron interferometers and neutron microscopes.