A process for preparing 2'-O-substituted guanosine comprising the steps of:a) reacting 2,6-diaminopurine riboside with a base of sufficient strength to remove a proton from the 2'-hydroxyl or 3'-hydroxyl of 2,6-diaminopurine riboside, and with a compound having the formula R--L wherein R is an aliphatic or alicyclic group and L is a leaving group, to form 2'-O-substituted-2,6-diaminopurine ribosid