The leaching properties of soluble components (Na+, NO -3, and NO-2) in synthetic bituminized wastes with various characteristics, such as salt content, salt composition, and bitumen matrix, were quantitatively evaluated by means of an aqueous leaching test. Micropores were formed in the bituminized waste by the leaching of soluble salts. The penetrability of bitumen and the salt content affected the size and the shape of the micropores observed by scanning electron microscopy. The linear relationship between leaching fraction of the soluble components and square root of leaching time was observed, and the diffusion coefficient of the soluble component was determined by Fick's law. The diffusion coefficients of ionic species of interest were similar to each other. Moreover, the diffusion coefficient agreed among bituminized wastes with the same salt content, even if salt composition and bitumen matrix were different. The diffusion coefficient of the component was proportional to the salt content in the synthetic bituminized wastes. It was found that the salt content in the bituminized waste was one of the key factors to determine the diffusion coefficient. © 2013 Atomic Energy Society of Japan. All rights reserved.