Some new liquid crystalline 2,5-disubstituted 1,3,3-thiadiazole derivatives incorporating a central group (-CH=N-) have been synthesized by treating 2-amino-5-(4-alkoxyphenyl)-1,3.4-thiadiazole with corresponding p-alkoxybenzaldehyde. Their structures have been characterized with Elementary Analysis, IR, (HNMR)-H-1 and MS, and their properties were determined by using DSC and Texture. All 12 compounds are enantiotropic liquid crystals, and most of them exhibit nematic mesomorphism and broad smectic C mesomorphic ranges. Several compounds show only the smectic C mesophase. By introducing an -OH group into the phenyl ring the thermal stability of the mesophases increases due to the formation of a hydrogen bond.