We report chronoamperometric measurements of the reduction of a monolayer of butanethiols chemisorbed on an Au(lll) single crystal electrode in 0.1M KOH. The reductive desorption of the butanethiol monolayer is found to follow a nucleation and growth mechanism. The mechanism of the reductive removal of the butanethiols changes smoothly from a progressive to an instantaneous nucleation and growth mechanism as the potential is stepped to more negative values. For the most negative applied potentials the reductive current decays at a slower rate than predicted from the model. We suggest that this is caused by the fact that the reduction process becomes limited by the diffusion of the thiols from the surface.