on-acoustic solitary waves in a weakly relativistic warm plasma at the critical phase velocity are investigated using reductive perturbation theory. The basic set of fluid equations describing the system is reduced to a renormalized warmion modified Korteweg—de Vries (mKdV) equation for the first-order perturbed potential and a renormalized linear inhomogeneous equation for the second-order perturbed potential. The stationary solution of the coupled equations is obtained. The cold-plasma limit is considered in order to rule