To shed light on dramatic differences between dl- and meso-1,3-bis(hydroxyphenylmethyl)benzenes in the solvent-free polycondensation, which takes place below the melting point of each monomer and gives the polyether, IR spectroscopic study of these monomers has been performed at variable temperature. The dl monomer proved to have stronger hydrogen-bonding networks in the crystalline state than the meso monomer, while these diastereoisomeric monomers showed quite identical IR spectra in the fusion state. These findings correspond with the polymerization behavior of these monomers; the dl monomer yields the higher molecular weight polymer in the solvent-free polymerization, while these two monomers are indistinguishable in the solution polymerization. The changes in the IR spectra with increasing temperature were found to take place all at once at the melting point of each monomer, supporting the differences between these monomers in the solvent-free polycondensation.