FT-IR and FT-Raman spectroscopic studies of the mesogens with the general molecular formula 1-(p-n-alkyl phenyl) 3-(p-n-alkyloxyphenyl) propane 1,3-diones having alkyl chain lengths C10, C11 and C12 are carried out. An analysis of the vibrational spectra of C10, C11 and C12 at room temperature strongly suggests that while C12 probably belongs to a highly ordered monoclinic or orthorhombic type of lattice, C10 and C11 belong to a hexagonal or triclinic unit cell, having lesser degree of lattice order. © 2001 OPA (Overseas Publishers Association) N.V. Published by license under the Gordon and Breach Science Publishers imprint, a member of the Taylor & Francis Group.