The process of solute movement into and out of the stationary phase or mobile phase. The C-term of the van Deemter equation is referred to as the mass transfer term. The faster the process of mass transfer, the better the efficiency of the column. In HPLC, mass transfer is the most important factor affecting column efficiency. It is increased by the use of small-particle packings, thin layers of stationary phase, low viscosity mobile phases, and high temperatures.