O-aryl-O, S-dialkylphosphorothioates, such as fully-protected adenylyl(3′→5′)-5′-thiouridine 11, O-aryl-O-ethyl-5′-thiouridyl phosphorothioate 24, upon treatment with an excess n-tetrabutylammononium fluoride in tetrahydrofuran-pyridine-water (8:1:1 v/v/v) underwent a scission of phosphorus-sulfur bond to give the corresponding O-alkylphosphoromonofluoridates 20 (74%), and 30 (75%). This facile, new preparation of alkylphosphoromonofluoridates has been found to be a general reaction which has been exemplified by the conversion of O-aryl-O, S-dialkylphosphorothioates 26, 27, 28 and 29 to the corresponding phosphorofluoridates 30 (79%), ( 31 + 32, together 85%), 33 (63%), and 34 (90%) The fully-protected adenylyl (3′→5′)-5′-thiouridines 12 and 13 were partially deprotected to 37 and 38, having a phosphorothioate linkage with a bridging sulfur [ribonucleoside-3′-O-PO 2-S-5′-ribonucleoside] in order to examine the stability of this internucleotidyl linkage vicinal to a 2′ -hydroxyl function. The 2′-O-protected adenylyl(3′→5′)-5′- thiouridines 37 and 38 were found to be as stable as thymidylyl(3′→5′)-5′-thiothymiifine 41. Removal of the 2′-O-protecting group from 37 or 38 however gave only a transiently stable adenylyl(3′→-5′)-5′-Thiouridine 39 which promptly decomposed through a phosphorus—sulphur bond cleavage, due to the nucleophilc attack by the vicinal 2′-hydroxyl group, both under mildly acidic and neutral conditions, to give the 2′,3′-cyclic phosphate 40 (∼75%), and 5′-thiouridine derivatives 18 and 19.