In this paper evidence of the negative effect of weathering on the sulphur removal by pyrolysis is reported. The study of the evolution of sulphur forms during the pyrolysis of a weathered coal has been conducted by chemical analysis, transmission Mössbauer spectroscopy and scanning electron microscopy with energy dispersive X-ray analysis. This study shows that in the pyrolysis of weathered coals some special chemical changes occur when compared with the pyrolysis of unweathered coals. In particular, no accumulation of organic sulphur in chars has been observed, however, inorganic iron sulphates formed by the oxidation of pyrites are decomposed above 500°C and yield complex sulphides which accumulate in chars, decreasing the efficiency of the desulphurization. These sulphides, because of their unreactivity, cannot be determined completely by chemical analysis, which implies an overcalculation of organic sulphur when calculated by difference.