Polyimide (PI) films coated on Cu films were treated under a high humidity condition. FT-IR RAS measurements have clarified the interface reactions of Cu with PI, the hydrolysis of imide functional group and formation of Cu carboxylate. From the calculation of RAS spectra measured at various incident angles. the diffusion of Cu carboxylate was confirmed from interface to the surface of PI. The diffusion length of Cu carboxylate ranges about 1 mum from Cu interface. The RAS result coincides with the ATR depth profile result and TEM-EDX image. Transmission electron microscope (TEM) images show that Cu diffuses mainly into the Cu carboxylate formation under our high humidity condition.