The thermotransport and diffusion of carbon in vanadium and V-Ti alloys, and the effects of divanadium and diniobium carbide particles on the thermotransport of carbon in vanadium and niobium were investigated. A radioactive tracer technique was used to determine the concentration profiles;The transport of carbon is toward the hotter regions of the sample, corresponding to a negative heat of transport, Q*, for carbon in both vanadium and V-Ti alloys. The magnitude of Q* for carbon decreases monotonically with increasing titanium content from -42.27 kJ/mol in vanadium to -13.97 kJ/mol in V-20.5 at .% Ti, and approaches that in (beta)-Ti. The activation energy for diffusion of carbon, however, increases almost linearly from 116.3 kJ/mol in vanadium to 188.5 kJ/mol in V-20.5 at .% Ti, and decreases to 94.6 kJ/mol in (beta)-Ti;Carbon thermotransport in niobium, as in vanadium, is toward the hotter regions, however, the direction of net carbon flux reverses when these samples are in a two-phase condition. Prolonged heating of an initially two-phase sample results in the development of a one-phase region in its hotter portion. The magnitudes of the apparent heat of transport, Q(,app)*, obtained from the one-phase region of such samples are greater than those obtained from the one-phase samples at steady state, i.e., Q*. The one-phase region in the sample expands upon continued heating at a decreasing rate, and the magnitude of Q(,app)* approaches that of Q*. It is shown that the solvus of an interstitial-metal system can be determined from the concentration of the boundary between the one- and two-phase regions of different samples run under various conditions. Microstructural observations of the behavior of carbides indicate that they act solely as sources and sinks for the dissolved carbon, maintaining local equilibrium between the matrix and carbide phases. The concentration profiles calculated using a mathematical model for transport of interstitial solutes in one- and two-phase alloys are in good agreement with the experimental results. The phenomenological and atomistic aspects of the one- and two-phase thermotransport of interstitial solutes in;metals, the models and mechanisms for the phenomena, and the experimental techniques employed are discussed; ('1)DOE Report IS-T-1149. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.