In the course of structural studies of aliphatic polyesters of the type [–O–(CH2)x–O–CO–(CH2)y–CO–]n, the molecular and crystal structures of polyethylene succinate) (x=2, y=2) and poly(ethylene oxalate) (x=2, y=0) were determined by interpretation of X-ray diffraction patterns and complementary study of infrared spectra.The crystallographic data are:Poly(ethylene succinate); a=7.60 A, b=10.75 A, c (fiber axis)=8.33 A, orthorhombic space group Pbnb-D2h10, four molecular chains pass through the unit cell.Poly(ethylene oxalate); a=6.44 A, b=6.22 A, c (fiber axis)=11.93 A, orthorhombic space group Pbcn-D2h14, two molecular chains pass through the unit cell.The chain conformation of poly(ethylene succinate) is T3GT3G, in which the two C(H2)–C(H2) bonds in the chemical repeating unit are G or G, where T means trans form and G and G are used to discriminate right-handed and left-handed gauche forms, respectively. The chain conformation of poly(ethylene oxalate) is T5GT5G, in which the C(H2)–C(H2) bond is G or G. There are some close similarities between the molecular and crystal structures of poly(ethylene succinate) and poly(ethylene oxalate), and their structures are quite different from polyethylene-like structures for the higher members (x=2, y≥4) of the ethylene glycol series. The high melting points of poly(ethylene succinate) and poly(ethylene oxalate) among the homologous polymers may be related to these molecular and crystal structures.