General yet simple description of chemical transport processes in non-isolated system is suggested. It is based on extended Teorell equation and just two fundamental parameters: physicochemical potential and Einstein's mobility. Using mobility it is possible to compare the rates of all major linear transport phenomena, including pressure-driven migration and also nonideal and multicomponent diffusion. Relationship with the Stefan-Maxwell approach and Onsager's linear thermodynamics is demonstrated and physical interpretation of both diagonal and off-diagonal phenomenological coefficients is suggested. Imposing boundary conditions for transport equations allows description of transport in homogeneous membranes caused by several concurrent driving factors, such as concentration, pressure, and voltage. Differences of barodiffusion, membrane filtration, and reverse osmosis are considered. For a porous membrane an expression for the pressure-driven volumetric flux through the pores as a function of mobility and pore size is derived. It is explained why hydraulic flow prevails in submicron pores and why diffusion is the dominant mechanism in reverse osmosis if the pressure difference is not too high. The theory naturally leads to the solution-diffusion model equations but does not need usual assumptions of constant pressure across the membrane and the pressure jump only at one surface. Internal pressure and mechanical stress gradients within a membrane exist and can be useful in a description of rheology of aging polymer membranes. A new equation for concurrent diffusion and hydraulic transport is derived and two possible molecular mechanisms leading to the Kedem-Katchalsky equations for reverse osmosis membranes are suggested. Finally, electrokinetic processes are described and their similarity to concentration- and pressure-driven transport is discussed. (C) 2009 Elsevier Ltd. All rights reserved.