Lactone ring formation in poly(alpha-hydroxyacrylic acid) (PHA) was investigated by means of H-1 and C-13 NMR. The stereoregularity of PHA was estimated as syndio-rich (ca. 86%) on the basis of methylene proton peak of the sodium salt, PHANa, in aqueous solution. On the other hand, analyses of the C-13 NMR spectra obtained as a function of the degree of dissociation (alpha) suggested that the lactone ring is formed at every other residue at half dissociation (alpha = 0.5). This specific way of lactone formation supposed for PHA is discussed in relation to the critical change in the dissociation behavior of the polyacid which has been found at alpha = 0.5.