Rates of mass transfer inside conical cavities machined in the wall of a vertical rectangular duct were measured by the electrochemical technique. Variables studied were solution flow rate, physical properties of the solution and cavity apex angle. The mass transfer coefficient inside the cavity was found to be less than the flat surface value, and decreased with increasing the cavity apex angle. Mass transfer data inside the cavity were correlated by the equation: Sh = 0.549Sc(0.33) Re-0.33 (h/d)(-0.524) Practical implications of the present results in determining the rate of diffusion controlled processes which may take place inside cavities, such as corrosion, electropolishing, electrochemical machining, electroplating, electroless plating, etching, etc. have been highlighted. (c) 2005 Elsevier B.V. All rights reserved.