Pyrimidine 4′-thionucleosides of the formula (I)
where B¹ is a pyrimidine base;
and either (a) R² is hydrogen and R³ is hydroxy or fluoro,
or (b) R² is hydroxy and R³ is hydrogen, hydroxy or fluoro,
or (c) R² is fluoro and R³ is hydrogen or hydroxy,
or (d) R² and R³ together form a carbon-carbon bond; and physiologically functional derivatives thereof and processes for their production. Preferably, the base B¹ is of the formula (II)
wherein Y is hydroxy or amino, monoalkylamino or dialkylamino; and X is halogen, alkoxy, alkyl, haloalkyl, alkenyl, haloalkenyl, alkynyl, amino, monoalkylamino, dialkylamino, cyano or nitro. The compounds are useful in the treatment of viral infections.