The production of carboxylic acids by partial wet oxidation of alkali lignin was studied experimentally. The factors influencing the different types of products, their yields and concentrations were investigated. Formic, acetic, succinic, oxalic, and glutaconic acid were the main identified products. Both the temperature and oxygen partial pressure are shown to have direct effects on the product yields whereas the lignin concentration has an indirect effect. At low lignin concentrations, the yield of products was relatively high. However, at higher concentrations, the yield decreased. By measuring the lignin molecular weight distributions, it is shown that this decrease is linked to repolymerization/condensation