The main objective of this paper is to study the thermal decomposition of waste products from leathers tanned with different tanning agents, from two different points of view: (i) thermogravimetric analysis and (ii) flash pyrolysis using a Pyroprobe device connected to a gas chromatograph with a mass spectrometer (Py/GC-MS). Both techniques allow us to characterize the samples by evaluating their potential differences regarding the decomposition process as well as by identifying the significant volatile compounds obtained depending on the tanning processes. The results have been treated using a multivariate statistical analysis method.