The Soil Moisture and Ocean Salinity (SMOS) mission is aimed at monitoring, globally, surface soil moisture and sea surface salinity from radiometric L-band observations. The SMOS radiometer relies upon a two-dimensional (2-D) synthetic aperture concept in order to achieve satisfactory spatial resolution performances for a minimal cost in terms of payload mass and volume. Counterparts of this advantage are reduced radiometric sensitivity and increased complexity. The performances expected from SMOS, in terms of measurement accuracy, spatial resolution, and revisit time, depend on many parameters, among which several are crucial for assessing the payload and mission configurations. Most prominent among those configuration parameters are the flight altitude, the length of the interferometer arms, the spacing between radiating elements, and the tilt angle of the antenna plane. Their selection has to be optimized, so as to satisfy both scientific requirements and main technical constraints. This paper describes the way the optimization was carried out during the SMOS phase A. After assessing the main drivers on instrument configuration from the science requirements, the goal was to find an optimal trade-off, minimizing technical challenges while fulfilling the science objectives. It was found that, even though salinity retrievals are the most challenging, soil moisture retrievals were the most demanding in terms of mission definition and that a configuration exists to satisfy the required retrieval accuracies. The obtained configuration was then checked against ocean salinity retrievals and found satisfactory.