The size and shape of micelles formed in aqueous mixtures of the anionic surfactant sodium dodecyl sulfate (SDS) and the nonionic sugar-based surfactant n-decyl â-D-glucopyranoside (C10G) at different concentrations of added salt have been investigated with small-angle neutron and static light scattering. Rather small prolate ellipsoidal micelles form in the absence of added salt and at [NaCl] ) 10 mM in D2O. The micelles grow considerably in length to large rods as the electrolyte concentration is raised to [NaCl] ) 0.1 M. In excess of nonionic surfactant ([SDS]/[C10G] ) 1:3) at [NaCl] ) 0.1 M in D2O, several thousands of Ångstroms long wormlike micelles are observed. Most interestingly, a conspicuously large isotope solvent effect was observed from static light scattering data according to which micelles formed at [SDS]/[C10G] ) 1:3 and [NaCl] ) 0.1 M in H2O are at least five times smaller than micelles formed in the corresponding samples in D2O.