Supported nickel catalysts are widely used in hydrocarbon steam reforming for producing hydrogen. Carbon deposition is a major cause of Ni catalyst deactivation. In this work, supported Ni and Ni/Rh catalysts were synthesized and tested for liquid hydrocarbon steam reforming. Carbon analysis on the spent catalysts illustrates that the carbon deposition is significantly reduced with inclusion of Rh. Extended X-ray Absorption Fine Structure (EXAFS) analysis indicates close interactions between Ni and Rh atoms. Density functional theory (DFT) results show that a Ni/Rh alloy is thermodynamically more stable than Ni and Rh alone at the synthesis and reaction temperatures. Adsorption, diffusion, and oxidation of carbon atoms over Ni and Ni/Rh alloy surfaces were examined by DFT calculations. Alloying with Rh reduces the stability of deposited carbon atoms and clusters over both terraces and steps of the catalyst. Moreover, Rh addition enhances the competition of oxidation reactions against carbon deposition by altering the relative diffusion rates and bond formation rates of the two processes.