Résumé : In this work, different thermal analysis methods have been used to study the co-pyrolysis of low-density polyethylene (LDPE) and motor oil. Two kinds of motor oil were considered, a fresh one and a used one. Through the comparison of experimental curves and so-called “theoretical curves”, high-resolution thermogravimetry experiments allowed highlighting interactions between the LDPE and each of the two oils, when they are co-pyrolyzed. While thermogravimetry coupled with mass spectrometry did not give any insights into these interactions, pyrolysis coupled to gaseous chromatograph and mass spectrometry allowed identifying aldehydes in the products of the co-pyrolysis of LDPE and each of the two oils. These aldehydes were not observed during the pyrolysis of the LDPE or the oils alone. On the basis of these results, various explanations for the formation of these aldehydes are proposed.