The composites made-up from renewable fillers and polymer matrix have drawn great attention due to the renewable nature, improved thermal and mechanical properties, environmental issues and most importantly to reduce dependency on fossil fuel resources. In this work, kraft lignin in its modified form (butylated lignin) was used to make composites with polystyrene successfully through bulk polymerization and high internal phase emulsion (HIPE) polymerization. The kraft lignin was first modified to butyrated lignin by esterification using 1-Methylimidazole as a catalyst in order to increase the compatibility as fillers with both monomer and polymer, which was further studied and verified through Hansen solubility parameter model. The thermal, mechanical, and structural properties of the lignin/polymer composites were systematically investigated. The incorporation of lignin in the composites could increase the modulus significantly and almost double (1,391 MPa) at 15 wt% of lignin loading as compared with bare composites. Excellent porous structure and mechanical properties are maintained with the lignin content as high as 10 wt% of the total foam mass. POLYM. ENG. SCI., 59:964-972, 2019. (c) 2018 Society of Plastics Engineers