Various conditions for synthesizing poly (1,3,4-oxadiazoles) from carboxylic acids, their hydrazides, and polyhydrazides, are investigated. Methods of cyclodehydration of polyhydrazides using phosphorus oxychloride, sulfuric acid, and certain other reagents are not of general utility for synthesizing poly (1,3,4-oxadiazoles). Vacuum-heating polyhydrazides gives crosslinked polymers. A new method of preparing poly (1,3,4-oxadiazoles) is offered. It is the reaction of hydrazine sulfate or dihydrazides of acids with dicarboxylic acids in oleum solution. Some chemical properties of the compounds synthesized are investigated.