  The collisional coupling of relativistic electrons and non-relativistic ions
in hot plasmas has been analysed. It is found that relativistic effects produce
a new feature: while the condition T_e<3T_i guarantees a stable collisional
coupling between electrons and ions in low-temperature plasmas, relativistic
effects shift the upper T_e/T_i boundary of stability to higher values.
Moreover, for sufficiently high temperatures, T_{e,i} > 75 keV, collisional
decoupling between relativistic electrons and ions becomes impossible.
