The conformational characteristics, as embodied in the unperturbed mean-square end-to-end distances ([r(2)](o)) and the characteristic ratios of the dimensions [C-n = [r(2)](o)/(n[l(2)]] are calculated for the para, meta, and ortho isomers of poly(ethylene phthalate)s: poly(ethylene terephthalate) (PET), poly(ethylene isophthalate) (PEI), and poly(ethylene phthalate) (PEP), respectively. Although each of these isomeric and partially aromatic polyesters has identical permissible conformations available to their ethylene glycol fragments, their connections through the ester bonds to the phenyl rings are quite distinct. In addition, for the ortho isomer (PEP), the close spatial proximity of the ester groups bonded to the same phenyl ring results in an interdependence of their orientations with respect to each other and the phenyl ring to which they are attached, unlike the independent orientations of ester groups in the para and meta isomers (PET and PEI). Conformational. energy calculations, dependent on the orientation of the ester groups in PEP, are used to characterize their rotational interdependence to modify the rotational isomeric state (RIS) conformational models for PET and PEI and thereby obtain an RIS model appropriate for PEP. This leads to calculated relative dimensions ([r(2)](o)) of 1.0:0.70:0.37 PET:PEI:PEP and characteristic ratios [C-n = [r(2)](o)/(n[l(2)])] of 4.13:4.67:2.49 PET:PEI:PEP. These results are discussed in an effort to obtain some understanding of the inherent static (or equilibrium) and dynamic flexibilities of the isomeric poly(ethylene phthalate)s.