Thermal desorption of covalently functionalized SWNT was followed using Raman, X-ray photoemission (XPS), and thermodesorption (TDS) spectroscopies. By functionalizing different sources of SWNT, we assess the thermal stability of phenyl- and methylene-SWNT derivatives in relation to the source diameter and helicity distribution. For all samples, broad desorption features were observed at similar to 600 K for the phenyl-SWNT and at similar to 500 K for the methylene-SWNT derivatives. In both cases, no influence on helicity and on diameter was observed for the range studied. The study shows that the stability of methylene addends on SWNT is inferior to that of the phenyl and proves that the main desorption pathway of phenyl addends is a phenyl-phenyl coupling reaction.