An improved process for preparing N6 -substituted aminopurine ribofuranose nucleosides. Compounds of this type are known to be usefull in the prepartation of compounds having activitity at adenosine receptors, e.g. Adenosine A1 receptor. The process comprises the step of reacting a 6-halopurine ribofuranose nucleoside with an amine in the presence of CaCO3 , wherein acid is added to the reaction mixture.