  We consider the phase transition of a string with tension $\eps_1$ to a
string with a smaller tension $\eps_2$ at finite temperature. For sufficiently
small temperatures the transition proceeds through thermally catalyzed quantum
tunneling, and we calculate in arbitrary number of dimensions the thermal
catalysis factor. At $\eps_2=0$ the found formula for the decay rate also
describes a break up of a metastable string into two pieces.
