The mass transport of electroactive compounds in a commercial SPE membrane was investigated using SPE composite microelectrodes. The diffusion coefficient and concentration of electroactive compounds in the membrane were measured by chronoamperometry and voltammetry. The experimental results indicate that electroactive compounds in the membrane diffuse as in a fluid electrolyte solution, except more slowly, but faster than in a recast SPE membrane; and the concentration of electroactive compounds dissolved in the membrane differs from that in bulk solution.