The application of Fourier transform infrared spectroscopy (FT-IR) to the characterization of materials in art and archaeology is evaluated. The diffuse reflectance accessory was used extensively and an infrared microscope was utilized for microscopic samples. The development and theory of diffuse reflectance FT-IR spectroscopy are given and a brief outline of previous use of infrared spectroscopy in archaeological and art conservation is included. The experimental procedures and sample handling used in the research are explained in detail. Diffuse reflectance spectra of several classes of organic materials available in antiquity are presented. The classes of organic materials include waxes, fats and oils, bituminous materials, resins, amber, shellac, pitch, gums and gum resins and proteins. The spectra of the reference materials are interpreted in the light of the known information on chemical structure. Several examples of archaeological specimens which have been characterized are included. Two large groups of modern materials, a group of plastic sculptures and a collection of early plastic objects were characterized. Areas for future work include and expanded reference collection of modern materials and the use of J-CAMP-DX programming language for interlaboratory exchange of data which is independent of the brand of spectrometer used.