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Sewage treatment plants (STPs) are important in densely populated megacities like Hong Kong to control the release of harmful pollutants from households and industries into the receiving water bodies and maintain water quality for supporting various beneficial uses. This study investigated the occurrence of the teratogenic retinoic acids (RAs) and their oxidative metabolites in sewage and sludge of six selected STPs that treat about 87% of all sewage in Hong Kong annually, and compared the removal efficiencies of these compounds from sewage between two major sewage treatment processes, i.e., chemically enhanced primary treatment (CEPT) and secondary (biological) treatment. The total concentrations of the studied RAs in influent, effluent and sludge from the six selected STPs were found between 21.5 and 33.1 ng/L, 12.0–20.4 ng/L, and 4.33–7.02 ng/g dry weight, respectively. The compounds were …