The aim of the study was to determine optical properties of vitrinite, subjected to temperatures within the range of 400-1200 degreesC for durations of 1-7 h, in an argon atmosphere. The samples of vitrinite concentrate (Vt content 90%, R-r = 1.17%) were inserted in the furnace at room temperature and subjected to a heating rate of 60 degreesC/min. To assess the effects of the experiment, random reflectance as well as apparent maximum and minimum reflectances of vitrinite were measured on the raw and thermally treated concentrates. Employing the method and computer programme of W.E. Kilby, the true maximum, intermediate and minimum reflectances of vitrinite as well as reflectance anisotropy and Reflectance-Indicating Surface (RIS) shapes were evaluated. Transformation of vitrinite subjected to heating is influenced more by the increase of temperature than the time applied. During the plastic stage (400-500 degreesC), vitrinite is subjected to substantial chemical and physical alteration. It is manifested by chemical decomposition, significant mass loss and mesophase appearance. Porosity, relief, reflectance and bireflectance of the vitrinite grains increase with heating. At the temperature of 1200 degreesC, random vitrinite reflectance reaches 6.62% with bireflectance of 3.46%. Mesophase at 1200 degreesC was characterised by mean reflectance equal to 9.25% and a bireflectance of 13.35%. The reflectance character of vitrinite changes from biaxial negative into biaxial positive-negative. However, the original RIS shape, resulting from the tectonic regime during the main phase of coalification, is not fully transformed until heated to 1200 degreesC. Reflectance character of mesophase is consistently biaxial negative. The internal structure of mesophase is thought to be better organised than the internal structure of vitrinite. (C) 2003 Elsevier Science B.V All rights reserved.