The present invention relates to a process for the preparation of 5-amino-3H-thiazolo[4,5-d]pyrimidin-2-one which is a useful intermediate in the preparation of certain thiazolo[4,5-d]pyrimidine nucleosides. The process comprises halogenating 2,4-diaminopyrimidine to form 2,4-diamino-5-halo-pyrimidine and then cyclocondensing the 2,4-diamino-5-halo-pyrimidine to form 5-amino-3H-thiazolo[4,5-d]pyrimidin-2-thione. The 5-amino-3H-thiazolo[4,5-d]pyrimidin-2-thione is then oxidized to afford the 5-amino-3H-thiazolo[4,5-d]pyrimidin-2-one.