Bromine seems to have similar behaviour to chlorine during thermal conversion: bromine forms high vapour pressure salts in the furnace, bromine contributes to high temperature corrosion of boiler steels and brominated dioxins and furans have been detected in combustion plant effluents. This paper presents a review on the natural origin, content and modes of occurrence of bromine in solid substances used as fuel. The most important natural source of bromine is the sea, by means of sea salt deposition and rainfall. In some marine algae bromine can be found up to several weight percentages in a large variety of organic compounds but only few mg kg(-1) in terrestrial plants where bromine is in the form of (mobile) bromide. Peat exhibits varying physical and chemical properties as a function of diagenesis, which facilitates the enrichment of bromine in comparison to chlorine. The total bromine content is in the order of 20 mg kg(-1) for inland peats and up to 200 mg kg(-1) for coastal peats. In coal, bromine content is inherited from the plant matter and peat. Typical content is < 20 mg kg(-1), but due to ground water influence bromine content can be one order of magnitude higher than that. In coal bromine is assumed to be bound mainly as anion sorbed on the coal organic surface pores. Shales are depleted in bromine.