Based on a series of well-defined oligomers of lactic acid with different end-groups, calorimetric measurements and vibrational spectroscopic studies have been carried out to examine chain conformation and chain packing. These studies yielded data that are directly related to the models used for conformational analysis of both crystalline and amorphous PIA chains. The previously proposed rotational isomeric model dominated by the trans-gauche'-trans (tg't) sequences has been confirmed. The linear relationship between enthalpy of fission and reciprocal of lamellar thickness for chain-extended crystals was used to calculate the equilibrium enthalpy of melting of the a crystalline form of PTA and the value was found to be in close agreement with the previously proposed value of 93 J/g reported by Fischer. Using the same technique, the enthalpy of melting for the alpha' phase has been determined to be 60 J/g. This confirms the value of 57 J/g reported by Kalish.