Abstract: A voltage-regulated coulometry on RDE is described. It shows that the electric quantity due to the reaction of soluble intermediate Ms, Q_(Ms), is proportional to the inverse square root of the rotation speed, ω~(-1/2). And the slope of the Q_(Ms)-ω~(-1/2) plot is proportional to the surface concentration of the soluble intermediate Ms. However, the qantity Q_(Mad), the electric quantity due to the reaction of adsorbed inter-mediate Mad is independent of the angular velocity of rotation of the electrode.