Abstract The well-known starch-iodine reaction was taken up to study the applicability of gel filtration to the problem of physical chemistry. Gel filtration by the frontal analysis was carried out, the following results being obtained. (1) Soluble starch- and amylose–iodine systems give elution volumes of iodine containing constituents (Vsi and Vai, respectively) distinctly smaller than that of iodine, which decrease with increasing concentration of iodine, C, and starch, approaching the elution volume of starch. This is a direct experimental evidence of starch–iodine complex formation. (2) An equation derived for Vsi vs. C and Vai vs. C relations holds satisfactorily. The concentration of free iodine in equilibrium with the complex is calculated by the equation. (3) Presence of iodide ion facilitates the complex formation, but its decomposition is instantaneous when it is isolated from free iodine. (4) Some iodide ions are found to be incorporated in the complex. (5) Amylopectin also forms a complex with iodine, but the amount of iodide incorporated is not large. (6) Potentiometric titration of starch with iodine exhibits a similar behavior to the gel filtration measurement.