The variations which occur in the thermal stability of the nematic mesophase as a function of the structure of the mesogenic moiety are considered here for the role played by three types of modifications. These are changes in the chemical nature of the mesogenic group, variation of the axial ratio of the rigid core and the presence of lateral substituents on the aromatic rings. Low molecular weight and polymeric liquid crystals of similar chemical nature have been investigated in order to see if parallel structure-property relationships exist between the two groups of compounds. In particular, starting from a mesogenic group built up of three aromatic rings connected by ester bonds, we have introduced the following modifications: (i) substitution of the central -COOC,H,OOC-with the -CH=N-N=CH-group;(ii) addition of two oxybenzoic end units to increase the length of the mesogenic moiety; (iii) introduction of two or more lateral methoxy substituents on the aromatic rings. The compounds were obtained by low temperature solution esterification between acyl chlorides and phenolic derivatives in the presence of a tertiary amine. Syntheses generally took place through the preliminary preparation of suitable intermediates. The thermal stability and the nature of the mesophases have been examined by different techniques. An interpretation of the results on the basis of the axial ratio and the strength of orientation dependent mutual attractions is attempted for model compounds. As far as polymers are concerned thermodynamic parameters follow the expected trend, if compared with those of low molecular weight analogues. Qualitatively models and polymers exhibit a similar dependence of mesophase stability on geometrical and electronic effects.