The mesomorphic properties of homologous series of N,N’-dialkanoyl-2,5-dimethyl-4-alkanoyloxy-1,3-benzenediamines and their 2-methyl-5-isopropyl and 2-methyl-5-t-butyl analogues have been studied. All the mesophases appearing in the nonanoyl to hexadecanoyl derivatives of the first series are metastable and nematic in type, By the replacement of the methyl group at the 5-position with an isopropyl or t-butyl group, nematic thermal stability is promoted. As a result, the nonanoyl to tetradecanoyl derivatives in the second series can exhibit a nematic phase with a temperature range of stable existence over 7 to 22 degrees C. The 2-methyl-5-isopropyl compounds, in which the ester alkyl group is different from the other alkyl groups, were also synthesized. The broadest temperature range of stable existence of a nematic phase of 31 degrees C is achieved when tridecyl groups are connected through the CONH groups and a octyl group through the COO group. The t-butyl group at the 5-position stabilizes not only the mesophase but also the crystalline phase; therefore, a nematic phase is detectable only for the undecanoyl to hexadecanoyl derivatives.