SUBSTANTIALLY PURE METHYLTHIONINIUM CHLORIDE PENTAHYDRATE FORM A IS PREPARED FROM METHYLTHIONINIUM CHLORIDE BY PHASE EQUILIBRATION OF SUSPENSIONS, CRYSTALLIZATION OR SOLVENT EVAPORATION, WHEREBY THE WATER CONTENT OF THE SOLVENT CORRESPONDS TO A WATER ACTIVITY OF AT LEAST 0.4 AT 25°C, AND CONTROLLED DRYING OF SAID METHYLTHIONINIUM CHLORIDE PENTAHYDRATE FORM A WITHIN ITS STABILITY RANGES OF HUMIDITY, PRESSURE AND TEMPERATURE.