The three-dimensional structure of the thermotropic copoly(ester carbonate) comprised of p-hydroxybenzoic acid (HBA), hydroquinone (HQ) and carbonate units has been investigated by X-ray methods, focusing on the copolymer containing a 75/25 HBA/HQ mole ratio. The fibre diagrams contain nonperiodic layer lines, and we have previously shown that these arise from a structure consisting of arrays of parallel chains with a completely random comonomer sequence. The chains have highly extended conformations, in which the carbonate groups are distorted from the minimum-energy planar structure, such that the 1,4-axes of the phenylene groups are all approximately parallel to the fibre axis. The X-ray data also contain a number of sharp equatorial and off-equatorial Bragg maxima due to the presence of three-dimensional order. We have first addressed the determination of the structure by calculation of the cylindrically averaged intensity transforms of single chains averaged over all possible sequences. We then considered the interference effects due to packing of the copolymer chains. The results show that the planes of successive phenylene rings, linked either by ester or carbonate groups, are mutually inclined by similar to 60 degrees. This leads to a pseudo-dimer repeat unit, which explains the existence of the layer line at Z = 1/13.1 Angstrom, between the equator and the first observed meridional maximum. The off-equatorial maxima are predicted when we require that short, random segments form ’crystallites’ by registration of the central monomer units. This registration is relatively minimal, in that there is a distribution of the centre of the sequences about the registry plane, with a standard deviation of similar to 1 Angstrom. It is seen that copoly(ester carbonate) chains with distorted carbonate groups can pack in a more ordered structure than is possible in the wholly aromatic copolyesters prepared from HBA and 2-hydroxy-6-naphthoic acid (HNA).