The effects of processing variables (time, temperature, and pressure) on the strength development during self-bonding of amorphous PEEK films were studied using a modified single lap-shear test. It was shown that the self-bonding strength developed isothermally at different bonding temperatures exhibits a linear response with the bonding time raised to the 1/4 power in agreement with Wool’s theory. DSC measurement of the crystallinity produced at different bonding conditions demonstrated that even though PEEK specimens contain the same amount of crystallinity, the resultant self-bonding strength is sensitively dependent on bonding history. C-mode scanning acoustic microscopy (C-SAM) was applied to define the effect of the processing variables on wetting of the bonding area during the bonding process. It was shown that, above a threshold pressure (< 17 psi), the degree of wetting depends weakly on time, but not on temperature. SEM analysis revealed that amorphous PEEK films are self-bonded by crystalline growth after diffusion and entanglement of the polymer chains across the interface. The crystalline growth rate across the interface is much higher at higher temperatures, leading to a higher self-bonding strength. The shear fracture surface observations also support the above result. The PEEK specimens showing the higher self-bonding strengths exhibit much denser striations and deeper dimplelike ductile patterns in the fracture surface, arising from much more crystalline growth across the interface.