A kinetio equation [Eqn.(6) in Chinese Text] of porous electrode containing slightly soluble active substanoe was derived.Reduction of porous mercuric oxide electrodes was studied with slow linear potential sweep method (sweep rate-6 m V/s).The current-potential curves of the porous red mercuric oxide electrodes thus obtained agree well with the above mentioned equation.But the experimental curves of the porous electrodes containing other kind of mercuric oxides(e.g., yellow mercuric oxide, Ag-doped mercuric oxide, and Tl-doped mercuric oxide) deviate from the equation, when overpotentials of the electrodes are greater than about 0.1 V.