We formulate a linear difference equation which yields averaged
semi-inclusive decay rates for arbitrary, not necessarily large, values of the
masses. We show that the rates for decays M \to m+\M' of typical heavy open
strings are independent of the masses M and m, and compute the ``mass
deffect''M−m−M′. For closed strings we find decay rates proportional to MmR(1−D)/2, where mR is the reduced mass of the decy products. Our
method yields exact interaction rates valid for all mass ranges and may provide
a fully microscopic basis, not limited to the long string approximation, for
the interactions in the Boltzmann equation approach to hot string gases.Comment: 35 pages, 3 figure