Codeposition of elements is an important phenomenon in the analysis and understanding of mass transfer in electrorefining of pyroprocessing. This study investigated the codeposition of lanthanides in molten salt. Mass transport experiments were conducted to examine the accuracy of the 1D electrorefining model, ERAD. Three lanthanides (La, Ce, and Gd) were selected because their standard reduction potentials are close to each other which facilitates codeposition. For the electrodeposition experiments, six electrochemical cells were used with varying ratios of lanthanides and the electrode potential ranging from 0.00 V to 0.15 V. This controlled the codeposition environment. The resulting electrodeposits were analyzed by ICP-OES and compared with the simulation results from ERAD. The ERAD input parameters were determined through electrochemical studies. Theoretical codeposition ratios were also analyzed using polarization curves. The study indicated that the current density of each element played a major role in its codeposition, as did thermodynamically determined reduction potentials. The overall results were comprehensively investigated and showed that ERAD can reasonably show the tendency of the codeposition results obtained from the experiments. With minor revision, it is expected that this 1D computer code could be effectively utilized for modeling the mass transport of the electrorefining process of pyroprocessing.