The invention concerns a process for preparng a thiazolo[3,2-a]benzimidazole of formula I ##STR1## in which formula n is 1 or 2, and R.sub.1 is hydrogen, lower alkyl, lower alkoxy, trifluoromethyl or halo, or a salt thereof which comprises dehydrating a compound of formula II, ##STR2## or a salt thereof wherein n and R, are a defined above, and COOR is an acid or ester function in the presence of an acid selected from sulphuric, sulphonic and phosphoric acid or mixtures thereof and if desired or required an inert solvent provided that if water is present then the amount of water is less than about 15% by volume of the acid, the reaction being carried out with heating if necessary.