This invention relates to a process for the preparation of an (p)-8-substituted adenosine-3',5'-cyclic phosphorothioic acid, or a salt or ester thereof, which comprises P-amidating 8-bromoadenosine-3',5'-cyclic phosphoric acid, and reacting the P-amidate with a base and with carbon disulphide to yield (p)-8-bromoadenosine-3',5'-cyclic phosphorothioic acid or a salt or ester thereof.