The present study was designed to investigate formation of possible carboxylic acids from oxidation of lignin by Fenton and UV-Fenton advanced oxidation processes (AOPs). The study was performed with model lignin monomers (syringaldehyde (SA), 4-hydroxybenzaldehyde (4-HBA), 2-methoxyphenol (2-MP) and 2,6-dimethoxyphenol (2,6-DMP)) and also with lignin isolated from eastern red cedar. Products in samples withdrawn from the reactor mixture at different oxidation stages were determined by GC-MS. Oxidation of one of lignin monomer (2-MP) was also performed with a different AOPs (UV/TiO2) for comparison of oxidation products. Depending on the oxidation stages, various carboxylic acids having aromatic and non-aromatic structures with mono- or di-carboxyl functional groups were determined. Performing oxidation of 2-MP with UV/TiO2 led to formation of 2-hydroxypropanoic acid as a low-carbon chain monocarboxylic acid which was not formed in Fenton and UV-Fenton. Oxidation products from the lignin fraction had mono- and di-carboxylic acid products in low percentages. (C) 2018 Elsevier Ltd. All rights reserved.