The lifetime of dipole-bound CH3CN- ions formed in K(np)/CH3CN collisions is measured using a Penning ion trap designed specifically for heavy ion storage. The data show that the decay of the trapped population is characterized by a single exponential that, when corrected for collision-induced loss, yields a mean ion lifetime tau similar to 85 mus. It is argued that the most likely process governing this lifetime is photodetachment induced by background thermal 300 K black-body radiation. (C) 2003 Elsevier B.V. All rights reserved.