The influence of adsorbed moisture on the ESR signal of the free radical in synthetic melanin is investigated. Water is found to correspond to 10% to 20% of melanin mass in the as synthesized state. Water removal is responsible for an increase in spin density induced by extended heating at temperatures above 60 degreesC. This enhanced spin concentration is stable at room temperature but can be quenched by rehydratation. A systematic study on the time dependence of the spin enhancement is presented along with a macroscopic model based on rate equations.