Thiofluorescein was synthesized in a preparative yield by successive demethylation of 3,3′-dimethoxydiphenyl sulfide to 3,3′- dihydroxydiphenyl sulfide and condensation of the latter with phthalic anhydride in H2SO4. The title compound was also obtained in a higher yield from 3,3′-dimethoxydiphenyl sulfide directly in a one-pot process combining the condensation in H2SO4 and demethylation in 2 Py·H2SO4. 2005 Pleiades Publishing, Inc.