Rates of mass transfer at hemispherical cavities machined in the wall of a vertical rectangular duct were measured by an electrochemical technique which involved measuring the limiting current of the cathodic reduction of K3Fe(CN)(6). Variables studied were solution flow rate, physical properties of the solution and cavity diameter. The mass transfer coefficient inside the cavity was found to be less than the flat surface value (the duct wall), and decreased with an increase in the cavity diameter. Mass transfer data inside the cavity were correlated by the equation:sh(c)=0.89(Sc.Re.-d(e)/d)(0.33)where Sh(c), and Sc and Re are Sherwood, Schmidt and Reynolds numbers, respectively; and d(e) and d are the duct equivalent diameter and cavity diameter, respectively. Practical implications of the present results in mass transfer limited processes such as diffusion controlled corrosion, electrochemical machining, electroplating and etching have been highlighted.