Intramolecular lactone ring formation of poly(a-hydroxy acrylic acid) (PHA) has been investigated as a function of pH with i.r. spectroscopy. I.r. peaks observed at 1772 +/- 14 cm(-1), 1724 +/- 4 cm(-1), and 1605 +/- 10 cm(-1) were assigned to C=O(lactone ring), C=O(COOH), and COO-, respectively. These i.r. peaks shifted to higher wavenumbers with decreasing pH. This shift was attributed to intramolecular hydrogen bonding, formation of lactone ring, and also to protonation of carboxyl group on the basis of frequency analysis by nb initio calculation. The respective molar fractions of lactone, COOH and COO- groups were estimated as a function of solution pH on the basis of the respective i.r. peak heights. According to the estimation, the fraction of lactone increases with decreasing pH, finally to cn. 65 mol% at pH 1. Ab initio calculation of the lactone ring formation energy supported the i.r. estimate.