Resumo em inglês Pyrohydrolysis is proposed for fossil fuels sample preparation for further fluorine and chlorine determination. Samples were heated during 10 min at temperatures up to 1000 °C. Water vapor was passed through the reactor and the volatile products were condensed and collected in NH4OH solution. Fluoride was determined by potentiometry using an ion selective electrode (ISE) and Cl by ICP OES and DRC-ICP-MS. The results are in good agreement with certified values and the pre (mais) cision is better than 10% (n = 4). Sample preparation by means of pyrohydrolysis is relatively simple, whereas chlorine and fluorine can be determined at low concentrations.