We demonstrate N-phosphorylation and N-thiophosphorylation of unprotected aminonucleosides in aqueous media. N-Phosphorylations using phosphoric chloride and N-thiophosphorylations using thiophosphoryl chloride were explored as functions of pH using 5′-amino-5′-deoxyguanosine as substrate. These reagents were compared to phosphodichloridate and thiophosphodichloridate ions, and the methodology was applied to other aminonucleosides. S-Alkylations of the nucleoside N-thiophosphoramidates were investigated as functions of pH and alkylating agent.