Improvement of primer adhesion to thermoplastic olefins (TPOs) by a low-temperature cascade are torch treatment was investigated. A cascade are was created with argon, and a treatment gas (air or methane) was added in the reaction chamber. It was found that the argon torch as well as the air/argon torch and the methane/argon torch can improve the primer adhesion to TPOs. Tape adhesion tests (ASTM method 3359-92a) demonstrated this improvement; a rating of "0" for untreated TPOs and "5" for air, methane, and argon torch-treated TPOs under certain operating conditions. Wettability of primer and of deionized water to TPOs were evaluated by contact angle measurements. TPO surface morphology was evaluated using scanning electron microscopy (SEM). The surface composition was characterized by using electron spectroscopy for chemical analysis (ESCA). The mechanism of improvement of primer adhesion to TPOs is discussed in this article. Primer adhesion was easily enhanced by the treatments for the soft and flexible TPOs (ETA-3041c and ETA-3101). Primer adhesion to hard and brittle TPOs (ETA-3183) was more difficult. Adhesion performance of primer to air, methane, and argon torch-treated TPOs was shown to have different mechanisms. For air plasma-treated surfaces, polar oxygen and nitrogen-containing groups were produced in the nonpolar surface layers, forming chemical bonds to TPOs; and plasma-etched pits spread all over the surface of TPOs into which the primer penetrated to form a mechanical interlock between TPOs and primer. For argon plasma-treated surfaces, the weak boundary layers (WBLs) of TPO surfaces were converted to a stronger layer by argon plasma, which resulted in enhancing primer adhesion to the surfaces of TPOs. The methane/argon torch allowed the deposition of a plasma polymer layer which replaces the weak boundary layer on untreated TPOs with a tightly crosslinked plasma polymer layer, and provides polar groups on the surface (by postplasma reaction of the residual free radicals with air).