Extreme environment applications require materials that have melting temperatures in excess of 3000 °C and that can retain good mechanical properties at high temperatures. Transition metal carbides (TMCs) are excellent candidates due to their high melting temperatures, high hardness, and low chemical reactivity. These materials display a duality of mechanical responses dependent on structure and temperature. This research aims at understanding the phase stability and deformation behavior of TMCs at compositions and temperatures that have received little investigation. A series of HfxTa1-xC compositions were computationally predicted, fabricated, and verified by experimentally identifying their phase formation, hardness, and dislocation behavior. Hardness values obtained via nanoindentation verified computational trends which predicted a modest rise in the Hf-rich ternary compositions. The presence of small amounts of Ta in Hf-rich ternary compositions yielded a change in slip system from the reported <110>{110} in HfC to <110>{111} commonly observed in TaC. To gain insight into the deformation and slip behavior of TaC and HfC at ultra-high temperatures, a thermo-mechanical testing apparatus was built for deforming specimens between 2100 °C to 2900 °C. Samples were resistively heated in the presence of a magnetic field to produce a non-contact Lorentz force. Greater deflection was observed for HfC up to 2300 °C attributed to differences in grain size. TaC deflection increased with rising temperature whereas HfC deflection decreased. This unexpected observation was discovered to be an artifact of plastic deformation that occurred during the preload. Mass transport and diffusional creep were found dominant with a preference for <110>{110} slip behavior observed for both carbides. To understand phase stability in the Nb-C system, a series of NbCx compositions were fabricated to span between the single-phase NbC and single-phase Nb2C with several compositions residing in multi-phase regions of the phase diagrams. Equiaxed grains formed for all compositions with those between ~ 0.56 to ~ 0.63 C/Nb exhibiting a lath–like microstructure as well. Additionally, a diffusion couple was processed near the same conditions to establish the phase transformations that lead to the observed microstructures. Carbon was observed to deplete from NbC and react with the Nb metal to form β-Nb2C.