Fully aromatic copolyesters with regular chain structure containing kinked monomer units were investigated by powder diffraction experiments combined with structure modelling. Temperature dependent X-ray measurements show the existence of low and high temperature solid state modifications of poly(p-phenylene isophthalate), (IPA-HQ)(n). From the X-ray scattering data, d-spacings were determined taking some experimental corrections into account. Lattice parameters as well as atom coordinates of lt-(IPA-HQ)(n) were calculated and optimised by means of modelling procedures based on force field constrained RIETVELD algorithms. From this, a monoclinic unit cell of lt-(IPA-HQ)(n) with a = 0.775 nm, b = 0.567 nm, c = 2.410 nm, beta = 91.8 degrees was found considering the chemical structure as well as interactions between the particles. The (LPA-HQ)(n) polyester is a typical representative of this kind of polymer with kinked monomeric, and hence is useful for further structural investigations.