A system for transdermal alcohol sensing to be worn near a skin surface of a user, including: an alcohol sensor; a microporous membrane; a housing coupled to the alcohol sensor and the membrane, defining a volume between the alcohol sensor and a first membrane side, and fluidly isolating the volume from a second membrane side opposing the first membrane side; an electronics subsystem electrically coupled to the alcohol sensor, operable to power and receive signals from the alcohol sensor; and a fastener operable to position the second membrane side proximal the skin surface.