We propose a method of detecting the presence of certain chemical and biochemical substances by virtue of the fact that they are, or have attached to them, either a dye molecule which acts as a photosensitizer for a holographic recording or erasing process, or another essential molecular component of the holographic recording material such as a monomer or a free radical generator. A recording material used in the process utilizes a photopolymer system consisting of a monomer and a crosslinking monomer, a free radical generator, a photosensitizer and, additionally, a polymeric binder when dry formulations are required and specifically lacks the substance to be detected. Subsequent holographic exposure records an interference pattern only where the complementary substance is present. Applications are broad ranging. Examples in diagnostics, printing, security, and environmental monitoring are given.