SUMMARYThe optical properties of vitrinite and exinite from a low‐rank coal (volatile matter = 37.3% dry ash free) oxidized with air on a fluidized bed at 300°C over periods up to 24 h have been studied. Reflectivities in air and oil of the unoxidized maceral areas within the particles and of the oxidized rims were measured at five wavelengths between 436 and 644 nm and the absorptive and refractive indices calculated using Beer's equation. The reflectivity spectra and the variation of the optical properties of the oxidized macerals with time of oxidation are discussed. It is concluded that the spectrum of oxidized exinite is a good parameter to indicate oxidation and that in low‐temperature oxidation experiments, when no oxidation rims appear, the inner part of the particles may not be oxid