5, 6, 7, 8-Tetrahydro-8-deaza-8-thiafolic acid (3) was synthesized as a diastereoisomeric mixture via thermal condensation of 5-hydroxyisocytosine (6) with diethyl N-[p-(2-amino-3-mercaptopropyl)aminobenzoyl]glutamate (10)after activation of the C(6)-position in 6 with N-bromosuccinimide/ethanol. The corresponding N5, N10-methylene derivative (5) was also prepared upon treatment of 3 with formaldehyde.