Microcapsules can encapsulate various chemical substances in their inner spaces, and it is thus possible to attain a controlled permeation of chemicals by using appropriate microcapsules. Because of their characteristics such as small size, huge total surface area, large inner volume, and stable membrane, microcapsules have found many applications in various fields from drug delivery through to the textile, petroleum, and pesticide industries. As the release rate from core-shell microcapsules is generally controlled by the rate of diffusion of the permeants across the thin microcapsule membrane, a faster response of the release rate to environmental stimuli may be expected as compared to crosslinked gels and microspheres. Therefore, core-shell microcapsules are suitable for stimuli-responsive controlled-release systems. Since the 1980s, environmental stimuli-responsive microcapsules have been investigated widely. These microcapsules control the release of their contents according to environmental stimuli. They are considered to be potentially useful as controlled-release systems, and especially so for drug delivery, because the target of a controlled drug delivery system is improved drug treatment (outcome) through rate- and time-programmed and site-specific drug delivery. [1] By encapsulation inside these microcapsules, chemicals or drugs can be released at a desired rate only when and/or where the release is needed. Modern environmental stimuli-responsive microcapsules have been reported to act in response to changes in temperature, [2-9] pH, [10-16] light, [17] external electric field, [18] redox conditions, [19] Ca2-ions, [20] and other stimuli, and these ™smartº capsules continue to gather increasing attention. In order to promote the applications of environmental stimuli-responsive microcapsules, development of microcapsules responsive to other signals remains essential. Herein, we report on the development of a molecular-recognition microcapsule for environmental stimuli- responsive controlled release: The release of the solute vitamin B12 (VB12) from the prepared microcapsules was significantly sensitive to the presence of Ba2+ ions in the environmental solution. In solution, the release of VB12 from the microcapsules was fast but slowed dramatically in the presence of BaCl2. The prepared microcapsules showed satisfactorily reversible and reproducible molecular-recognition stimuli-responsive release characteristics.