A series of 9-[2-(phosphonomethylthio)alkoxy]purine analogues of recently reported antiviral 9-[2-(phosphonomethoxy)alkoxy]purines (Duckworth et al., 1991) has been prepared and evaluated as antiviral agents. 9-[2-(Phosphonomethylthio)ethoxy]guanine (8) has potent activity against herpesviruses, and (R)-9-[3-hydroxy-2-(phosphonomethylthio)propoxy]guanine (37) has potent and selective activity against visna virus and good activity against HIV-1.