The radiosonde measurement programme of the boundary layer during the Hydrologic Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahel) is described. Typical examples of boundary layer evolution at two sites for wet and dry periods are shown. The Central Site was consistently dryer and warmer than the Southern Site and also reached higher boundary layer depths in the dry period in October. A brief analysis of surface flux variability is used to determine the errors associated with using a single average value for boundary layer input. Budget calculations using a mixed layer model show that advection plays an important role in the early morning budgets of heat and moisture. In the dry period subsidence also contributed to boundary layer heating.