The rate of mass transfer between the wall of a tube and a liquid flowing laminarly through the tube has been studied with the copper dissolution technique. The experiments revealed a significantly higher rate of mass transfer than predicted by the theoretical solution of the Graetz-Nusselt problem. The cause for this deviation was traced to natural convection induced by an increase in liquid density near the wall of the tube. An improvement to the copper dissolution technique is suggested that is shown largely to eliminate natural convection. (C) 2003 Elsevier Science Ltd. All rights reserved.