Results of the polymorphism investigation of three nematic liquid crystalline mixtures made of four compounds belonging to the thioester homologous series: 4-n-pentylphenyl-4'-n-alkoxythiobenzoate (in short nOS5, where n=4,5,6,7 stands for the number of carbon atoms in the alkoxy chain) are presented. Three out of them 4OS5. 5OS5 and 6OS5 possesses only enantiotropic nematic phases whereas 7OS5 possess a monotropic smectic C phase. The principal component of the mixtures was 4OS5 and the following mixtures: 4OS5/5OS5, 4OS5/6OS5 and 4OS5/7OS5 were prepared. The transition temperatures of all compounds and the binary mixtures were determined by means of DSC calorimetry and polarizing microscopy methods. For all concentrations linear dependence of T., was observed. The widest temperature range of the nematic phase (from -6 degreesC to 86 degreesC) was obtained for the 4OS5/7OS5 system. For this system a tricritical point at cooling the mixture containing 63% of 7OS5 was obtained. For all examined mixtures the eutectic point is observed on heating.