This is the first study that focused on the homopolymerization of itaconic anhydride (IAH) during melt radical grafting of poly(lactic acid) (PLA) at 190 degrees C. Although neglected by many authors, in the present study, radical-induced IAH homopolymerization, self-induced IAH homopolymerization, and IAH isomerization to citraconic anhydride were studied. The yield of investigated IAH-based side reactions was determined by Fourier transform infrared spectroscopy, and reaction by-products were extracted from raw PLA-g-IAH and characterized by thermogravimetric analysis (TGA) and size exclusion chromatography (SEC). It was found that the highest concentration of IAH and L101 favors IAH homopolymerization and affects the PLA grafting reaction mechanism dramatically.