The mechanism of dispersion of solute in gel chromatography using various Sephadex gels was quantitatively studied. In order to simplify the mathematical treatment, non-adsorptive low-molecular weight substances such as NaCl and glucose were chosen as samples. A pulse response experiment was carried out in a column. The longitudinal dispersion coefficient and the diffusion coefficient in gel phase were determined separately by applying the moment method to the elution curve. Then, their contribution to the column efficiency characterized by HETP was studied. Particularly, the effect of gel phase diffusion was examined in detail. The gel phase diffusion coefficient was apparently much smaller than the molecular diffusion coefficient. Consequently, it was revealed that gel phase diffusion played a much more important role in gel chromatography than what was expected by other investigators.