Infra-red and Raman spectra of 5,5′-bis-isoxazole-h4 and -d4 in the crystal state and in solution have been measured and the dichroism of the infra-red bands in polycrystalline samples has been determined. The data obtained are consistent with a trans planar structure in the crystal. In the liquid state and in solution, the molecule exists in a single distorted configuration, as shown by the spectra at different temperature and by the value of the dipole moment. An almost complete assignment of the fundamentals in the crystal has been obtained.