The adhesion strength between poly(ethylene terephthalate-co-isophthalate) (co-PET) and tin-free steel (TFS) was investigated by varying the lamination temperature using a blister test. As the lamination temperature increased, the adhesion strength increased rapidly at the beginning and then saturated. The main reason for the increase of adhesion with the lamination temperature was an improved degree of wetting. This was manifested by the fact that the variation in the degree of wetting was very similar to that of the adhesion strength. The degree of wetting was dependent on the melt viscosity of co-PET. The results of differential scanning calorimetry (DSC) and rheometrics dynamic spectrometry (RDS) showed that the high extent of the crystalline phase increased remarkably the melt viscosity of co-PET, leading to incomplete wetting. When the co-PET/TFS joint was immersed in hydrochloric acid solution, the pattern of the front penetration line and the rate of penetration of hydrochloric acid solution into the co-PET/TFS interface were different, depending on the lamination temperature and the direction of penetration. The wet adhesion strength was found to increase with increasing lamination temperature because the size of voids at the co-PET/TFS interface resulting from incomplete wetting decreased.