The dissolution of rare earth oxides in molten fluorides is a critical step in the preparation of the corresponding rare earth metals by oxide-fluoride electrolysis. However, quantitatively understanding the nature of dissolution, especially in the case of molten salts, is usually difficult to be achieved by postmortem characterization. In this paper, the dissolution behavior of Nd2O3 particles in molten fluorides was studied via in situ observation with confocal scanning laser microscopy. Combining direct observation with thermodynamic analyses on the oxide dissolution, the rate-limiting step(s) and the effects of parameters like temperature, salt type, and composition on the dissolution rate are identified. This study provides a methodology to estimate the dissolution kinetics of rare earth oxides in molten fluorides during their primary and secondary processing.