Novel analogues of the anti-HIV-1 agent TSAO-T, [1-[2',5'-bis-O-(tert- butyldimethylsilyl)-beta-D-ribofuranosyl]thymine]-3'-spiro-5"-(4"-amino- 1",2"-oxathiole-2",2"-dioxide) and its 3-methyl counterpart TSAO-m3T were obtained by modifications at positions 2' or 5' of the sugar moiety. These compounds were evaluated for their inhibitory effect on HIV-1 and HIV-2 replication in cell culture. Introduction of new groups at the 5'-position (i.e. esters, benzylether and silylethers) resulted in compounds that were either inactive or less active than the parent compounds (TSAO-T and TSAO-m3T). Attempts to introduce small silyl ether groups at this position were not successful since these products decomposed during purification. Similar modifications at the 2'-position had a much less pronounced influence on the anti-HIV-1 activity.