Nucleation and transport of a condensable trace species across subcooled laminar/turbulent boundary layers are investigated. Spontaneous homogeneous nucleation is found to occur for surfaces maintained at temperatures much below the dew point. The embryos so formed provide condensation sites for heterogeneous nucleation. The effect of boundary layer condensation is always to diminish the mass transfer of trace species. There is an intermediate temperature range where the decrease in vapor deposition is not fully compensated by the ensuing Brownian/thermophoretic deposition of particulates so that the combined particle and vapor mass transfer decreases with surface subcooling. For laminar flow, a vapor-free zone is created adjacent to the subcooled surface by direct vapor condensation on the entrained particulate matter. The mass transfer then occurs solely due to paniculate deposition mechanisms. For turbulent flow, vapor mass transfer persists even at low surface temperatures. A significant decrease in net deposition flux is observed partly due to turbulent back diffusion of particles into the superheated zone and, more importantly, due to strong turbulent convection.