The reaction between lactic acid and diphenylmethane diisocyanate was studied by H-1 NMR, C-13 NMR, and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy. The structure of the oligomers formed was also investigated and supported by using the postsource decay method. Analysis of the products revealed that, besides linear chain copolymer formation, branching and homooligomerization of lactic acid also took place. A mechanism is suggested for the cooligomerization processes.