The thermal stability of highly transparent and conducting polyaniline-Nylon 6 (PAn-N) composite films doped with Various protonic acids such as hydrochloric acid (HCl), benzenesulfonic acid (BSA), sulfosalicylic acid (SSA), and rho-toluenesulfonic acid (TSA) was investigated at different elevated temperatures under air atmosphere. Two different degradation kinetic processes of electrical conductivity were found depending on the species of protonic acids. The conductivity degradation of PAn-N composite films doped with SSA and TSA were found to obey first-order reaction kinetics, while that of the other dopants was found to follow multiorder kinetics. Spectroscopic studies revealed that the thermal de-doping process of doped composite films, which reversed the doping process, took place without major structural modifications of the polyaniline in the composite system but was not completely reversible. From the differential scanning calorimetry (DSC) study, the crystal structure of the composite films was found to be affected by the formation of polyaniline in matrix polymer and depended on the types of dopant species.