PROCESS FOR PREPARING COMPOUNDS OF THE BENZOTHIOXANTHENE SERIES, WHEREIN CIOMPOUNDS OF THE 1-(2''-AMINOPHENYLMERCAPTO)-NAPHTHALENE SERIES OR OF THE 1-PHENYLMERCAPTO=8-AMINONAPHTHALENE SERIES ARE REACTED IN TERTIARY AMINES WITH COMPOUNDS YIELDING NITROSYL GROUPS AND THE DIAZNIUM COMPOUNDS SO OBTAINED ARE HEATED. THIS PROCESS IS SUITED FOR ALL COMPOUNDS WHICH CONTAIN THE BENZOTHIOXANTHENE RING, INDEPENDENTLY OF THEIR SUBSTIUENTS. THE PRODUCTS OBTAINED BY THIS PROCESS HAVE A MARKEDLY HIGHER PURITY AND ARE OBTAINED IN A HIGHER YIELD AS COMPARED TO OTHER PROCESSES.