The dissociation rate constant of a microcanonical ensemble of endohedral Na@C-60(+). formed by gas-phase collisions, is determined to be 94000 s(-1) when the internal excitation energy reaches 49.1 +/- 0.5 eV. A model that includes radiative cooling and a distribution of energies, accurately reproduces the yield of Na@C-60(+) from its appearance at 29.6 eV to its disappearance above 55 eV, Assuming an Arrhenius factor of v(o) = 5 x 10(19) s(-1) we derive a dissociation energy of 10.17 +/- 0.02 eV (statistical error). The abruptness of the breakdown graph implies a lower limit for v(o). (C) 2002 Elsevier Science B.V. All rights reserved.