Both thermoplastic polyesters, poly(butylene-terephthalate) (PBT) and poly(butylene-2,6-naphthalate) (PBN), have been observed to undergo solid-solid phase transitions between alpha and beta crystalline polymorphs. Fiber repeat distances determined by X-ray diffraction in the beta polymorphs of PBT and PBN are larger than those observed in their a polymorphs. This has generally been attributed to an extension of the butylene glycol portion of both polyesters, where the O-CH2-curved right arrowphi-CH2-CH2 and (C=O)-O-curved right arrowpsi-CH2-CH2 bonds are transformed from phi = G to S and psi = G to T or S to T conformations, respectively, when PBT or PBN transform from their a to their beta polymorphs. However, this interpretation is not consistent with solid-state C-13 NMR observations made on the alpha and beta crystalline forms of PBT and PBN. In both instances, only very small differences (0.4-0.8 ppm) in the resonance frequencies of the butylene carbon resonances are observed between the C-13 NMR spectra recorded for both polymorphs. Similar small differences in the aromatic carbon resonance frequencies are observed between their alpha and beta polymorphs. Thus, we conclude that the extension of both PBT and PBN chains, as they are transformed from their alpha to their beta crystalline polymorphs, is more likely a consequence of an increase in the coplanarity of their ester groups and their phenyl or naphthyl rings, rather than conformational extensions of their butylene glycol fragments. A search for PBT and PBN conformations, both of which match the fiber repeat distances observed for their alpha and beta crystalline polymorphs and are consistent with the closely similar C-13 chemical shifts observed in both of their crystalline phases, lends further support to this suggestion.