Axisymmetric spreading of a liquid drop containing a soluble surfactant on a smooth solid substrate is numerically investigated for the case in which surfactant mass transfer between the interface and the bulk liquid is sorption/kinetic controlled. The fastest spreading rate is achieved by drops with O(1) values of Biot number for which the rate of surface convection is comparable to the sorption rate, and the surfactant molecules transferred to the interface are effectively convected to the contact line region. (c) 2006 Elsevier Inc. All rights reserved.