The work in this thesis involves four parts: Part one: Synthesis of 2,5-disubstituted-1,3,4-thiadiazoles: For the synthesis of the titled compounds the following sequence of steps (step -1- to step -8- ) were followed. Step-1- : Synthesis of 2,5-dimercapto-1,3,4-thiadiazole [1]. It was prepared by cyclization of hydrazine hydrate with carbon disulfide in alkaline medium. Step-2- : Synthesis of 2,5-dihydrazino-1,3,4-thiadiazole [2]. Hydrazinolysis under refluxing of compound [1] afforded the titled compound. Step-3- : Synthesis of 2,5-di(aryl hydrazone)-1,3,4-thiadiazoles [3-12]. Ten 2,5-di(arylhydrazone)-1,3,4-thiadiazoles [3-12] were prepared by condensation reactions of compound [2] with appropriate aromatic aldehydes. Step-4- and -5- : Synthesis of 2-phenyl-5-chloromethyl-1,3,4-oxadiazole [14]. Methyl benzoate was condensed with hydrazine hydrate to yield benzoyl hydrazine [13] (step -4-), which in turn cyclized through condensation-dehydration reaction with chloroacetic acid to yield 2-phenyl-5-chloromethyl-1,3,4-oxadiazole [14] (step -5-). Step-6-: Synthesis of 2,5-di-[5-phenyl-1,3,4-oxadiazole-2-thiomethyl]-1,3,4-thiadiazole [15]. The titled compound was synthesized by condensation of compound [1] with two equivalents of compound [14] through (SN2) reaction. Step-7- : Synthesis of 2,5-(dithio acetic acid)-1,3,4-thiadiazole [16]. The reaction of compound [1] with two equivalents of chloroacetic acid furnished the titled compound [16]. Step-8-: Synthesis of 2,5-di-[5-amino-1,3,4-thiadiazole-2-thiomethyl]-1,3,4-thiadiazole [17]. Condensation of compound [16] with two equivalent of thiosemicarbazide in the presence of phosphorus oxychloride afforded compound [17]. Part two: Characterization: the synthesized compounds [1-17] have been characterized on the basis of their (F.T.IR and UV-Vis) spectral data, the results were discussed in this part to give the final conclusion for the structures given to these compounds. Part three: Photostabilization of PVC films by 2,5-di(aryl hydrazone)-1,3,4-thiadiazoles. 2,5-diaryl hydrazone-1,3,4-thiadiazoles were used as additives for the photostabilization of PVC films, the photostabilization were monitored with irradiation time by measuring the carbonyl and polyene indices (Ico and Ipo) values using 0.5% wt/wt concentration of these additives. The following trend is obtained for the photostabilization effect of PVC films in the presence of these additives: [6],[10],[5],[8],[9]and[7] According to the experimental results obtained several mechanisms were suggested depending on the structure of these aryl hydrazones. Part four: Biological Activity. Some of the synthesized compounds were screened for their antibacterial against two strains of bacteria. But approximately two or three of the compounds exhibited worth mentioning activity.