A new type of organobentonite foundry binder composed of a composite of bentonite (SN) and poly(acrylic acid) (PAA) was analyzed using thermal analysis (TG-DTG-DSC) and pyrolysis gas chromatography mass spectrometry (Py-GC/MS). The temperature range in which the composite retains its binding properties was identified using thermal analysis of the composite and its components. Results showed that the thermal decomposition process is complex and involves physicochemical transformations that are mainly reversible at temperatures in the ranges of 20–100 °C (related to evaporation of solvent water) and 100–230 °C (related to intermolecular dehydration). The decomposition of PAA chains occurs between 230 and 300 °C, while complete decomposition of PAA and formation of organic decomposition products takes place at 300–500 °C. Dehydroxylation of montmorillonite (MMT) in bentonite begins at about 500 °C, which leads to a drastic structural transformation. An endothermic effect associated with the remodeling of the mineral structure was observed on the DSC curve in the range of 500–750 °C. The produced SN/PAA composite was found to be thermostable during degradation in both oxidative and inert atmosphere, similar to the starting bentonite, and even maintained over a relatively higher and wider temperature range compared to organic binding materials used. At the given temperatures of 300 °C and 800 °C, only CO2emissions occur from all the examined SN/PAA samples. There is no emission of compounds from the BTEX group. This means that the proposed binding material in the form of the MMT-PAA composite will not pose a threat to the environment and the workplace.