Halogenation of s-triazolo [2, 3-a] pyrimidine and its 5-methyl derivative with chlorine or bromine gave the corresponding 6-halo derivative (IIIa, IIIb, IVa, and IVb), but in the case of 5-hydroxy-7-amino-and 5, 7-dihydroxy-s-triazolo [2. 3-a] pyrimidines, reaction of bromine gave 6-bromo derivatives (VIIb and VIIIb) while that of chlorine gave 5-dichloro-acetamido-s-triazole (IX) instead of 6-chloro derivatives (VIIa and VIIIa). 2-Halo-1, 3-dicarbonyl compounds were condensed with 5-amino-s-triazole (X) into 6-chloro-s-triazolo-[2, 3-a] pyrimidines (VIa, VIIa, and VIIIa), but 6-bromo derivatives were not obtained due to resinification. Ultraviolet spectra of 19 kinds of 6-halo-s-triazolo [2, 3-a] pyrimidines were measured and compared with those of the corresponding original compounds.