An electrochemical technique which involved measuring the limiting current of the cathodic reduction of potassium ferricyanide was used to study the rate of mass transfer inside a cubical cavity machined in the wall of a vertical rectangular duct. Variables studied were side length, physical properties of the solution and flow rate of the solution. The mass transfer coefficient was found to decrease with increasing cavity size; in all cases, the mass transfer coefficient inside the cavity was less than that at the duct wall. Mass transfer data inside the cavity were correlated by the equation Sh(c) = 0.525 (Sc Re d(e)/L)(0.33). Comparison of the present results with the results obtained using other cavity geometries shows that cavity geometry plays an important role in determining the rate of mass transfer inside the cavity.