Thiazolo[3·2-a]benzimidazole (I) and several of its derivatives were synthesized by annelation of the thiazole ring to a benzimidazole. The intermediate 3-hydroxy-2,3-dihydrothiazolo[3·2-a] benzimidazoles were identified and the 3-position of the hydroxyl group was established by oxidation to 3-(2H)-thiazolo[3·2-a]benzimidazolone. Tautomerism of the 3-hydroxy-2,3-dihydrothiazolo[3·2-a]benzimidazoles was also investigated.3-Hydroxy-2,3-dihydrothiazolo[3·2-a]benzimidazole and the corresponding 3-methyl compound were found to undergo nitrogen acetylation and rearrangement under acetylating conditions.