A low-rank coal cleaned by heavy-liquids separation was treated by 1 M KOH solution at room conditions for a maximum of 2 h. The desulfurization mechanism in terms of total sulfur was well described by the model of unreacted-shrinking core in a homogeneous coal particle of unchanging size during process development. The external diffusion stage might be omitted when modeling this process. A chemical reaction component was considered during diffusive flow of KOH through particle pores, too. It was possible to remove 50% of total sulfur after 2 h of treatment, although three-quarters of achieved conversion happened at first 30 min.