The NMR residual quadrupole coupling of certain nuclei is introduced to determine the surfactant aggregate shape at solid surfaces, The major advantage of the NMR method is the sensitivity of the residual quadrupole coupling to the curvature of the surfactant aggregate interface. Being insensitive to the adsorption density, the NMR method represents a good supplement to other adsorption experiments. As an illustrative example, we discuss sodium dodecyl sulfate adsorbed on positively charged alpha-Al2O3-at the adsorption plateau of the isotherm. Using the NMR results only, we can reject a model with a continuous, flat surfactant bilayer or prolate micellar aggregates at the surface. However, three other geometries can account for the observed residual quadrupole coupling; long, ribbon-like aggregates (axial ratio in the range 2-3.5), oblate micelles (axial ratio ca. 6-12), or a porous bilayer (pore axial ratio 1-4).