  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 $M
m_{R}^{(1-D)/2}$, where $m_{R}$ 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.
