The phase behaviour of PBO3O4, a liquid crystalline polyetherester derived from 4-hydroxy-1,1'-biphenyl-4-carboxylic acid and the etherdiol 4-(3-hydroxypropoxy)butan-1-ol, has been investigated by differential scanning calorimetry (DSC), real-time synchrotron X-ray diffraction, optical microscopy and solid-state C-13 NMR. The results reveal an interesting polymesomorphism in PBO3O4, since, on cooling from the melt, a low-order smectic mesophase is obtained first, followed by a smooth transition into a slightly more ordered mesophase, and a final transition to a phase with a relatively high degree of order, stable at room temperature. The subsequent melting shows enantiotropic behaviour but, interestingly, the more ordered phase presents a considerably low undercooling. The 13 C NMR results in the solid-state of the ordered phase show that a single T-1p(H), relaxation time (and a single line shape for each particular carbon signal) is exhibited by PBO3O4. This fact, plus the low undercooling, suggests the formation of a highly ordered mesophase, instead of a three-dimensional crystal, which is the case of the corresponding polyester with the same spacer, or of the polyetherester with an all-methylene spacer. Additional experiments about the phase behaviour of the starting monomer for the synthesis of PBO3O4, reveal that a highly ordered mesophase is also obtained for the monomer, with a diffractogram rather similar to that exhibited by the ordered mesophase of the polymer. (c) 2006 Elsevier Ltd. All rights reserved.