<p>Poly(phenylene oxide) (PPO) exhibits excellent dielectric properties, making it an ideal substrate for high-frequency, high-speed copper-clad laminates. The phenolic hydroxyl group at the end of PPO plays a key role in its reactivity. Accurately quantifying the phenolic hydroxyl content in PPO is essential but challenging. In this study, we proposed a method for measuring the phenolic hydroxyl content of PPO using differential UV absorption spectroscopy. In alkaline solutions, the phenolic hydroxyl in PPO completely ionizes to form phenoxide ions, leading to a significant increase in UV absorbance at approximately 250 and 300 nm. Notably, the differential UV absorbance at approximately 300 nm was directly proportional to the phenolic hydroxyl concentration. Using 2,6-dimethylphenol as a standard, a calibration curve was established to relate the phenolic hydroxyl concentration to differential UV absorbance at approximately 300 nm, providing a precise and straightforward method for phenolic hydroxyl quantification in PPO with distinct advantages over conventional techniques.</p>