The thermotropic phase behavior of P3O4B, a polybibenzoate with an asymmetric oxymethylenic spacer, has been analyzed by DSC, real-time synchrotron X-ray diffraction, and optical microscopy. The results show that the following phases are found on cooling: isotropic melt, SA mesophase, Sc mesophase, and three-dimensional crystal. The subsequent melting reveals the enantiotropic character of P3O4B since the inverse phase sequence is found. Additionally, a clear recrystallization process is observed on melting, prior to the crystal-Sc transition. The thermotropic properties of P3O4B are compared with those of its analogue polyester with an all-methylene spacer, poly(octamethylene p,p'bibenzoate), P8MB, and other polybibenzoates with oxymethylenic spacers. Compared with P8MB, significantly lower transition temperatures are found for P3O4B due to the higher flexibility and asymmetry of the spacer. The lowering is especially important for the formation of the final crystal structure, so that the mesophase temperature window is considerably enlarged in P3O4B, allowing to observe the transition S-A to S-C. Moreover, the comparison of the thermotropic properties of P3O4B with those of other polybibenzoates with oxymethylenic spacers reveals that P3O4B is the only one able to crystallize at the usual cooling rates of the calorimeter.