Poly(L-lactide) (PLLA), prepared in a tin octoate-catalyzed reaction, was modified with various types and amounts of peroxides in the melt during mixing in an extruder or in a kneader. The modified material was analyzed using size exclusion chromatography (SEC), X-ray photoelectron spectroscopy (XPS), a differential scanning calorimeter (DSC), and thermogravimetric analysis (TGA) and by microscopic studies. Hydroperoxides, peroxy acid, peroxy ester, dialkanoyl peroxide, diaroyl peroxide, and dialkyl peroxydicarbonate stabilized the melt during mixing. Nonstabilizing peroxides were dialkyl and diaralkyl peroxide. The binding energy for the electrons in the Sn 3d level increased for the stabilized PLLA. Thus, the stabilization mechanism was deactivation of the species that catalyzes the reversible esterification reaction. The decomposition temperature of the peroxide-modified polymer increased, and the crystallinity decreased. However, annealing the peroxide-modified poly(L-lactide) resulted in almost the same degree of crystallinity as that of the unmodified polymer. This behavior was due to a slower crystallization rate for the stabilized PLLA.