Abstract Phase transformations in squaric acid (H 2 C 4 O 4 ) have been investigated by thermogravimetry and differential scanning calorimetry with different heating rates β . The mass loss in TG apparently begins at onset temperatures T di =245±5 °C ( β =5 °C min −1 ), 262±5 °C ( β =10 °C min −1 ), and 275±5 °C ( β =20 °C min −1 ). A polymorphic phase transition was recognized as a weak endothermic peak in DSC around 101 °C ( T c + ). Further heating with β =10 °C min −1 in DSC revealed deviation of the baseline around 310 °C ( T i ), and a large unusual exothermic peak around 355 °C ( T p ), which are interpreted as an onset and a peak temperature of thermal decomposition, respectively. The activation energy of the thermal decomposition was obtained by employing relevant models. Thermal decomposition was recognized as a carbonization process, resulting in amorphous carbon.