With four kinds of mobile phases, methanol-water, ethanol-water, 2-propanol and acetoritrle water (all cortairing 0.1% trilfu roacetic acid), the displacemetn between soltue and solvent in RPLC wsa proved to be universal in frontal analysis (FA), Based on the measured Z value in usual RPLC to be a corstant and the quarntitative determinnation of methanol increment in mobile phase in FA. the stoichiometric displacement (SD) betwwen insulin and methanol was directly proved by the experment. The SD was also proved to ocur only on about the bonded phase layer (BPL) without any dynamic problem of mass transfer, while in FA, the SD firstly occurs on the sufaace of the BPL and then graduallly sinks into the deeper sites comparied with a dynamic problem. Although the displaced solvent by the same solute is less in the formet case, the SD in independent, of how deep of the splute enters the BPF. In addition, the numbers of the adsorbed layer on the adsorbent sur-face, but also on the exent of the complete removal of the displace-able solvent in the BPL. The physical funda mental of the SD and the methodology for investigation were also discussed.