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Abstract. Pinholes and cracks in metal-oxide-coated multilayer PET food packaging films decrease their barrier performance, reducing the shelf life of the products packaged. These defects tend to have dimensions in nanometer to micrometer range and are present at random locations in the films, making it challenging to analyze them. In this study, we used an improved agar gel-based oxygen sensor to locate the defects in two metal-oxide-coated films (A and B). We filled the film pouches with water and processed them in a pilot-scale retort system for 40 min (R40) at 121 °C (target lethality (F₀): 6–9 min for solid food products). Afterwards, the processed pouches were cleared of water, filled with the oxygen sensitive gel, resealed and stored at 23±2 °C and 40 °C for 90 days. The yellow colored gel changed to blue with oxygen ingress during storage at the defective locations. Samples stored at 40 °C showed more blueness than 23 °C. Also, R40-processed gel-filled pouches showed more color change than unprocessed samples due to increase in their oxygen transmission rates (OTRs) after thermal processing. Film A showed a significantly higher (P<0.05) OTR than film B after processing, correlating well with the intensities of color change. The microscopic analyses showed that film B had fewer pinholes and cracks than film A, resulting in a lower OTR and color change. The gel showed low syneresis at both 23 and 40 °C as well. Overall, this method can help in reducing the design cycle time for developing improved coated PET films.