Nicotine in mainstream cigarette smoke is predominantly present in the particulate phase. Interestingly, however, the deposition efficiency of smoke particles in the respiratory tract is less effective than is the nicotine retention. In the literature, four nicotine deposition mechanisms are identified: (a) direct gas deposition, (b) evaporative gas deposition, (c) particle deposition with evaporation, and (d) particle deposition with diffusion. In this article we present a physically motivated fundamental model to address nicotine deposition mechanisms (b) and (c) from the vapor phase. The model incorporates nicotine mass transport through estimates for the diffusion time across the epithelial layer and the time for nicotine vapor diffusion from the gas volume to the tissue surfaces in the tracheobronchial and pulmonary regions of the respiratory tract. The model comprises four mass transfer processes for nicotine at the surface of the respiratory tract epithelium: (1) conversion of free base nicotine from protonated nicotine; (2) free base nicotine transport across the epithelium; (3) free base nicotine evaporation; and (4) diffusion of free base nicotine vapor from the surface gas layer into the airway lumen. Results of the nicotine mass transport model suggest that the principal mechanism of nicotine delivery to the lung is by direct deposition of particles to the alveolar fluid lining, followed rapidly by evaporation into the lumen and then gas diffusion back to the surface as nicotine depletes in the surface layer through its transport across the epithelium.