The polycondensation of D,L-lactic acid upon microwave irradiation was studied. The results of polycondensation by means of microwave were compared obtained from conventional heating of lactic acid at 100 degreesC, and it was found that the reaction proceeds with much. higher rate upon microwave irradiation. The oligomer mixtures formed were investigated by means of matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS). The molecular mass of the poly(lactic acid) formed under microwave irradiation was found to increase with irradiation time, and the formation of cyclic oligomers after 20 min of reaction time was also revealed.