Chemical changes in the structure of organic matter of coking coals during storage modify their thermoplastic properties and behaviour during carbonization. As a result, the anisotropic carbon structure of the metallurgical cokes produced and their physical properties are altered. In this work, the weathering behaviour of 10 bituminous coals of different geographic origin, rank and thermoplastic properties, used as components in the preparation of industrial coking blends for coke manufacture, was studied by means of Gieseler plastometry and Fourier transform infrared (FTIR) spectroscopy. These coking coals were stored in piles at the Instituto Nacional del Carbon (INCAR) open stockyard for a period of time of up to 7 months. Special attention was paid to the relationship between the relative amount and type of aliphatic hydrogen (semi-quantitatively evaluated by FTIR), and thermoplastic properties. Depending on the nature of the coking coal, a different response to natural weathering can be expected. Thus, the results showed that there is a direct link between a decrease in methylene groups and a loss of fluidity in the weathered coals, resulting in a decrease in anisotropic carbon of the resultant cokes with weathering time. In addition, the rate of anisotropic carbon loss induced by weathering could be associated with the rank parameters of the initial coals. (C) 2003 Elsevier Science B.V. All rights reserved.