The mass transfer between one substance or phase and another caused by simultaneous convection and molecular diffusion. It can involve the mass transfer between a fluid in motion and a surface, which may be either a solid or an immiscible liquid. The convective mass transfer coefficient relates the molar mass flux of a species to the concentration difference between the boundary surface concentration and the concentration of the diffusing species in a moving fluid. The coefficients k_G and k_L refer to the gas and liquid phases, respectively, and are related to the properties of the fluid, the dynamic characteristics, and the geometry of the system. They are often presented as a product with area through which mass transfer takes place as k_Ga and k_La. In this form, they are useful when considering gas transfer as bubbles, particularly in biological systems for oxygen transfer since oxygen is usually a limiting factor in a bioreactor.