Our aim in this paper is to make a comparative study of the mesogenic properties of liquid crystalline compounds derived from 6-n-alkoxy-3-pyridinecarboxaldehydes, 5-n-alkoxy-2-pyridinecarboxaldehydes and 4-n-alkoxybenzaldehydes. The relationship between molecular structure and mesogenic properties was determined using spectro-scopic techniques (U.V. and1H NMR) and semi-empirical methods of calculation (MNDO). The results have proved to be of interest in that a relation can be seen to exist between the dipole moment of the molecules and the net partial charges of the atoms and the type of mesophase formed.