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This thesis considers certain factors whose influence on the optical properties of coal macerals heated over a range of temperature and under varying conditions, have not been examined in detail. Reflectivity measurements in two media have allowed derivation of the fundamental optical parameters, refractive and absorptive indices, which, along with bireflectance, have been related to changes in the molecular structure of the heated minerals. The factors studied here are heating rate, prolonged constant heating below the decomposition point, the effect of mixing vitrinites and sporinites before carbonisation, as compared with the optical properties of each maceral carbonised separately, the effect of pressure on carbonised vitrinite and finally heating at elevated temperatures in the range 1000° to 2500°.