Abstract: A device for releasing a volatile substance into the environment in a controlled manner comprises a reservoir of substance-absorbent material, for storing the substance, that is encapsulated in an envelope, at least a portion of which comprises a permeable material having porosity at least equal to ultramicroporosity. The permeable envelope portion has a greater affinity for the substance than does the reservoir material. The remainder of the envelope comprises a material impermeable to the substance. Accordingly, the substance permeates through the permeable envelope portion to be released therefrom in vapor form into the environment.