The thermal decomposition of the byproducts of the biodiesel process was studied by thermoanalytical methods. Deoiled algae cake and jatropha seed deoiled cake were pyrolyzed and the catalytic effects of silica supported iron catalysts (Fe/FSM-16 and Fe/SBA-15) and magnetite (Fe3O4) were tested. The evolution profiles of the decomposition products as well as the thermal stability of the samples were determined by thermogravimetry/mass spectrometry (TG/MS). The formation of the volatile products was monitored by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). The composition and the amounts of the gaseous products changed significantly in the presence of the silica supported iron catalysts: the yield of hydrogen and carbon monoxide considerably increased above the decomposition temperature of 400 degrees C. Both.silica supported iron catalysts had important effects on the yield of the products originating from carbohydrates and lignins. The formation of anhydrosugars and phenolic compounds was hindered, while the evolution of aromatic and aliphatic hydrocarbons was enhanced. Fe/FSM-16 proved to be more efficient than Fe/SBA-15 and Fe3O4 catalysts. The thermal decomposition of the protein content of the samples resulted in the formation of 2,5-diketopiperazines and smaller molecules (e.g., ammonia). The silica supported iron catalysts had a special effect: their presence promoted the reaction of fatty acid esters and ammonia resulting in the formation of alkyl nitriles during the thermal decomposition.