A class of 2-aryl-4,5-dihydroxy-6-carboxypyrimidines of formula (I): wherein Ar is an optionally substituted aryl or heterocyclicgroup; as well as compounds of formula (I) which are derivatized at one or more of the 4-hydroxy, 5-hydroxy or 6-carboxy groups; and tautomers thereof, and pharmaceutically acceptable salts or esters thereof; and inhibitors of viral polymerases, especially the hepatitis C virus (HCV) polymerase enzyme