The nitration of 2,5-diphenyl-1,3,4-thiadiazole is investigated and the results are compared with the nitration results of 2,5-diphenyl-1,3,4-oxadiazole. The variation of isomer ratios with variation in nitration media occurs in the thiadiazole series, but not to the extent it occurs in the oxadiazole series. As the nitrating agent is changed from nitric acid alone to nitric acid in sulphuric acid, and then to nitronium tetrafluoroborate, the proportion of meta-nitrated products increases at the expense of para-nitrated compounds. The significance of these results, and those in other related phenyl heterocycles, are discussed. The attempted monobromination of 2-phenyl-5-(p-nitrophenyl)-1,3,4-oxadiazole is described. Previous attempts always resulted in polybromination of the phenyl ring. We were able to restrict the reaction to dibromination, the first bromine entering the ring in all the three positions in the ratio of ortho:meta:para = 28.5: 28.5: 43. The second bromine entering was ortho or para to the first bromine already present in the phenyl ring. Synthesis of (1,3,4-oxadiazole-2,5-diyl)dibenzoic acids and thiadiazole analogues of these acids is undertaken by oxidation of the corresponding ditolyl-oxadiazoles and -thiadiazoles. Previously described routes to these acids (some of them patented procedures) did not give satisfactory yields.