  We study kinetic master equations for chemical reactions involving the
formation and the natural decay of unstable particles in a thermal bath. We
consider the decay channel of one into two particles, and the inverse process,
fusion of two thermal particles into one. We present the master equations the
evolution of the density of the unstable particles in the early Universe. We
obtain the thermal invariant reaction rate using as an input the free space
(vacuum) decay time and show the medium quantum effects on $\pi+\pi
\leftrightarrow \rho$ reaction relaxation time. As another laboratory example
we describe the $K+K \leftrightarrow \phi$ process in thermal hadronic gas in
heavy-ion collisions. A particularly interesting application of our formalism
is the $\pi^{0}\leftrightarrow \gamma +\gamma$ process in the early Universe.
We also explore the physics of $\pi^{\pm}$ and $\mu^{\pm}$ freeze-out in the
Universe.
