The objective of this paper is to investigate natural convection driven by two buoyancy sources, such as heat and mass, in vertical boundary layers. Starting from the integral equations and using scale analysis, we derive the different asymptotic flow regimes encountered with different buoyancy forces and diffusion coefficients. Each type of how is characterized by a set of scaling relations yielding the velocity, temperature and concentration distributions inside the fluid, and the heat and mass transfer coefficients. All our results are perfectly corroborated by numerical investigations in a wide range of parameters.