  We consider the structure of mildly relativistic shocks in dense media,
taking into account the radiation and pair loading, and diffusive radiation
energy transfer within the flow. For increasing shock velocity (increasing
post-shock temperature), the first important effect is the efficient energy
redistribution by radiation within the shock that leads to the appearance of an
isothermal jump, whereby the flow reaches the finial state through a
discontinuous isothermal transition. The isothermal jump, on scales much
smaller that the photon diffusion length, consists of a weak shock and a quick
relaxation to the isothermal conditions. Highly radiation-dominated shocks do
not form isothermal jump. Pair production can mildly increase the overall shock
compression ratio to $\approx 10$ (from $4$ for matter-dominated shocks and $7$
of the radiation-dominated shocks).
