The stability of the interface between indium-tin-oxide (ITO) and poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) in polymer light-emitting diodes (p-LEDs) is crucial for device performance. Oxidation of the polymer by oxygen diffusing from the ITO anode limits the device lifetime. To address this, an organic hole-injecting film, PEDOT:PSS, is introduced between the ITO and the emissive polymer, leading to improved lifetime and luminous efficiency. However, Rutherford backscattering (RBS) studies show that the ITO/PEDOT:PSS interface is not stable. In as-prepared samples, 0.02 at.% indium was found in the PEDOT:PSS film due to etching of the ITO during spin coating. Annealing in a nitrogen atmosphere at 100°C for 2500 h increased the indium concentration to 0.2 at.%. Exposure to air caused faster degradation, with indium reaching a saturation concentration of 1.2 at.% after several days. This degradation is attributed to the strong acidic nature of PEDOT:PSS, which etches the ITO. The hygroscopic nature of PSS also plays a role, as water absorption forms an aqueous acid environment, facilitating ITO etching and transport of etch products through the PEDOT:PSS film. In samples with a PPV film on top of PEDOT:PSS, a sharp decrease in indium concentration was observed at the PEDOT:PSS/PPV interface, indicating that etch products are trapped in the PEDOT:PSS layer. The ITO/PEDOT:PSS interface is highly sensitive to air, with prolonged exposure leading to significant degradation. The study highlights the importance of interface stability in p-LEDs and the need for careful handling and protection against environmental factors.The stability of the interface between indium-tin-oxide (ITO) and poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) in polymer light-emitting diodes (p-LEDs) is crucial for device performance. Oxidation of the polymer by oxygen diffusing from the ITO anode limits the device lifetime. To address this, an organic hole-injecting film, PEDOT:PSS, is introduced between the ITO and the emissive polymer, leading to improved lifetime and luminous efficiency. However, Rutherford backscattering (RBS) studies show that the ITO/PEDOT:PSS interface is not stable. In as-prepared samples, 0.02 at.% indium was found in the PEDOT:PSS film due to etching of the ITO during spin coating. Annealing in a nitrogen atmosphere at 100°C for 2500 h increased the indium concentration to 0.2 at.%. Exposure to air caused faster degradation, with indium reaching a saturation concentration of 1.2 at.% after several days. This degradation is attributed to the strong acidic nature of PEDOT:PSS, which etches the ITO. The hygroscopic nature of PSS also plays a role, as water absorption forms an aqueous acid environment, facilitating ITO etching and transport of etch products through the PEDOT:PSS film. In samples with a PPV film on top of PEDOT:PSS, a sharp decrease in indium concentration was observed at the PEDOT:PSS/PPV interface, indicating that etch products are trapped in the PEDOT:PSS layer. The ITO/PEDOT:PSS interface is highly sensitive to air, with prolonged exposure leading to significant degradation. The study highlights the importance of interface stability in p-LEDs and the need for careful handling and protection against environmental factors.