New and artificially-aged leather were adopted as the degradation reference samples for archaeological leather. A combination of technologies, specifically thermogravimetry-Fourier transform infrared spectrometry (TG-FTIR) and pyrolytic gas chromatography-mass spectrometry (Py-GC/MS) were employed as the analytical methods to study the degradation processes, mechanism and products of archaeological leather. The experimental results show that both the archaeological and the new leather samples undergo similar thermal degradation processes, which can be divided into two stages. Free water volatilization from the leather samples occurred in the first stage and thermal decomposition of collagen of the leather samples occurred in the second stage. The main chemical reactions during the thermal decomposition process include dehydration, decarboxylation, deamination, dehydrogenation and side chain cleavage of amino acids. The main products due to the thermal decomposition include water, carbon dioxide, ammonia, diketopiperazine, isocyanic acid, hydrocyanic acid, pyrrole and methane, etc. Besides, collagen, tannin and lipids of the archaeological leather samples were degraded or lost during the degradation process, resulting in their significantly lower relative percentages than those of the new leather samples. The combination of TG-FTIR and Py-GC/MS is a powerful method for studying the mechanism of degradation of archaeological leather. This combination can also be employed to evaluate the degree of degradation of archaeological leather.