Leather-wood fibreboards are innovative composite materials, which combine together the high mechanical properties of wood with the superior fire behaviour properties of leather. This study deals with the understanding of the combustion mechanism of the wet-white leather panel. During burning, an overlay coating-like surface is formed on top of a foamy structure that creates the heat transfer barrier. The FT-IR spectroscopy results of the leather show the rearrangement of the proteins and the formation of an increasing amount of acid groups when the exposure to hot gun at over 530°C was prolonged. These acid moieties can react with amino groups of other peptide chains, building a protective polymer network which hinders the oxygen to reach the core of the panel. Simultaneously, the gases produced during rearrangement cannot easily leave the material, producing a foamy structure which slows down the heat transfer to the core of the material. The Py-GC-MS analysis shows that the gases produced by the wet-white leather-type protein-based boards were amino-aromatic compounds like the diketopiperazine (DKP), which do not burn easily. The combination of the effects of (i) formation of the overlay coating-like surface, (ii) establishment of the foamy structure, and (iii) degassing of DKP explains the outstanding fire properties of leather and wood-leather fibreboards.