Name given by John Knox to the following phenomenon: At a certain column length, a sample injected into the center of the packed bed spreads by radial diffusion but never reaches the column wall, where wall effects can cause band broadening. Knox showed that a sample peak collected in the exact center of the column exit displayed a higher efficiency than a sample peak collected near the wall. The infinite diameter effect depends on column length, internal diameter, particle size, and mobile phase properties. Very seldom applied in HPLC.