Aqueous solutions of the ionic surfactant sodium sulfopropyl octadecyl maleate (SSPOM) have been studied by small-angle neutron scattering (SANS) to characterize the micelles formed in the presence of 25 mmol/L NaCl at 50 degrees C. The external scattering contrast is varied by changing the composition of the solvent mixture from D2O to H2O and yields a reliable value for the volume of the monomer. The experimental scattering curves are best described by a system of interacting and polydisperse prolate ellipsoidal micelles with an aspect ratio of 1.15. The polydispersity is small, amounting to about one-tenth of its equivalent sphere radius of 3.27 nm. A mean aggregation number of about 200 is determined, confirming recent estimates from light scattering and time-resolved fluorescence quenching. The obtained quantitative data are in agreement with the theory of micellization and show that simple packing considerations are also useful for uncommon surfactants possessing a complicated conformational structure.