Partial wet oxidation (PWO) of kraft black liquor (KBL) and wheat straw black liquor (WSBL) was carried out at 170-230 degrees C. PWO was conducted under oxygen gas only or in the presence of iron catalysts or hydrogen peroxide as an additional oxidant. Unlike the catalysts, increasing temperature enhanced the PWO process in general. Lignin concentration decreased 57-64% and weight-average molar mass of lignin from 2.7-3.3 kDa of original black liquors to 1.5-1.8 kDa after PWO at 210-230 degrees C. Unlike in KBL, presence of hemicellulose in WSBL was responsible for increasing total acid concentration from 16 g/L to 30 g/L in PWO at 230 degrees C. A two-stage process comprising lignin precipitation before subsequent PWO decreased chemical oxygen demand of the liquid fraction by 38-41%. Lignin was fractionated into a solid fraction and a water-soluble fraction which showed 21-26% lower polydispersity compared to lignin in the original black liquors.