High performance polymers such as PEEK are currently considered unsuitable for the encapsulation of biomedical implants, partly because of a lack of a strong and durable bonding technology. Strong autohesive bonds were achieved between semi-crystalline PEEK surfaces using a treatment in RF plasma with a mixture of methane and oxygen (CH(4)/O(2)) gases. The surfaces were bonded under pressure above the glass transition temperature (T(g)) and below the melting point (T(m)). The treatment with CH(4)/O(2) gases produced significantly higher bond strength than untreated samples and samples treated with CH(4) only. The CH(4)/O(2) plasma treatments generated a higher polar component of surface energy than observed for the untreated surfaces and also higher than that observed for surfaces plasma treated in CH(4) alone. XPS analysis of surfaces treated with CH(4)/O(2) plasma for more than 3 min showed that oxygenated nano-films were deposited that contained C-O groups and these showed the highest bonding strength. Free radical induced cross-linking of the deposited nano-film is proposed as a mechanism to explain the high bond strength achieved.