\Ammonium perchlorate is the most widely employed oxidizer for composite solid propellants. When exposed to atmosphere, it absorbs moisture and agglomerates. It is usually vacuum dried in order to avoid this agglomeration. When ammonium perchlorate that has been exposed to atmosphere for a certain period of time, is used in making a composite solid propellant, the burning rate is different because of the change in particle size distribution due to its agglomeration. This change in burning rate will change the thrust-time profile from that of what it is designed for. As one goes to a finer ammonium perchlorate particle size this problem becomes more evident. Experimental studies aimed at reducing the agglomeration of ammonium perchlorate by coating it with activated charcoal. Ammonium perchlorate coated with 1% activated charcoal showed almost no agglomeration, even when the particle size of ammonium perchlorate is approx. 1 mm. The burning rates also remained unchanged when ammonium perchlorate coated with 1% activated charcoal was employed in propellant composition, after it has been exposed to the atmosphere for a period of 1 h.