A method of purifying ammonia borane is provided whereby crude ammonia borane is dissolved in a basic aqueous solution, the solution is heated to decompose and precipitate impurities found in the crude ammonia borane, and the impurities are separated by filtration. The aqueous solution containing dissolved ammonia borane is then cooled to a temperature of from 10° C. to −10° C., to precipitate the ammonia borane, which is recovered and dried to yield a high-purity product.