The melt polycondensation of D,L-lactic acid was studied by the MALDI-TOF MS method at temperatures ranging from 100 to 220 degreesC. At low temperature, i.e., up to 120 degreesC, linear, and at higher temperature both linear and cyclic oligomers were formed. The ring-chain equilibrium was investigated in detail and the concentration-based equilibrium constants (K,) and their dependence on the degree of polycondensation and on the temperature were determined. It was also found that the molecular size distribution of the linear chain oligomers obeys to "the most probable distribution". A detailed investigation also showed that the ring size distribution of the cyclic oligomers follows the Jacobson-Stockmayer theory.