The synthesis of 4'-ethynyl-2'-deoxy-4'-thioribonucleosides was carried out utilizing an electrophilic glycosidation in which 4-ethynyl-4-thiofuranoid glycal 16 served as a glycosyl donor. Electrophilic glycosidation between 16 and the silylated nucleobases (N-4-acetylcytosine, N-6-benzoyladenine, and N-2-acetyl-O-6-diphenylcarbamoylguanine) was carried out in the presence of N-iodosuccinimide (NIS), leading to the exclusive formation of the desired beta-anomers 29, 33, and 36. Anti-HIV studies demonstrated that these 4'-thio nucleosides were less cytotoxic to T-lymphocyte (i.e., MT-4 cells) than the corresponding 4'-ethynyl derivatives of 2'-deoxycytidine (44), 2'-deoxyadenosine (45), and 2'-deoxyguanosine (46). Comparison of the selectivity indices (SI) was made between 4'-thionucleosides (32, 41, and 43) and the corresponding 4'-oxygen analogues 44-46 by using the reported CC50 and EC50 values. In the case of cytosine and adenine nucleosides, comparable SI values were obtained as follows: 32 (545) and 44 (458); 41 (>230) and 45 (1630). In contrast, 4'-ethynyl-2'-deoxy-4'-thioguanosine 43 was found to possess a SI value of >18200, which is 20 times better than that of 46 (933).