An electrochemical method was used to measure the mass transfer coefficient in packed beds containing spherical particles. The electrochemical reaction was reduction of ferricyanide ions on stainless steel ball bearings in the presence of potassium nitrate supporting electrolyte. The correlation found in this study was ,D = 2.944 Re-0.5~4 / ~ )-~ where 1.8 < Re < 27, 2.98 < L/dp < 7.45, and Sc ----1421. The mass transfer coefficient for randomly packed beds containing up to five different sizes of spheres (0.398-0.952 cm diam) was correlated by the above equation when the average particle diameter was determined by 1 dpm --where dpi is the particle diameter of size i, and xi is the volume (or weight fraction) of particles of size i in the packed bed. It was found that when the bed packing was segregated into two parallel flow zones, each having different particle sizes, then the average mass transfer coefficient was less than the value given by the correlation above.