Carbon deposits that grow on coke oven ceramic brickwork are formed by cracking reactions of volatile products. Their structure depends on the type of volatile matters, the side of the oven where the sedimentation occurs, the free space temperature distribution, the contact time between hot gases and brickwork and the type of coke oven. The presence of the carbon deposits in a coke oven chamber influences its exploitation and life prolongation. On the one hand it has a positive effect on the gas-tightness of the ceramics and prevents the raw coking gas from escaping, but on the other hand it accelerates the degradation of ceramic elements. The aim of this study was to characterize the carbon deposit structure by fractal dimension and to assess the possibilities of usage of this parameter for determination of the thermo-dynamic stability. The fractal dimensions calculated for colourful microscopic images taken in magnifications of 25x and 200x were compared with conventional techniques like X-Ray diffraction, Raman spectroscopy, reflectance measurement, and texture analysis. It was found that there is a good correlation between fractal dimensions and the results of the listed methods. However, the place of deposit sampling has an influence on the convergence of that correlation. The deposits from the inside of the oven have similar optical character, while the deposit from the ascension pipe is different, due to the different thermal and chemical conditions of deposition. (C) 2014 Elsevier B.V. All rights reserved.