Mixed micelles of dodecyltrimethylammonium bromide (DTAB) and didodecyldimethylammonium bromide (DDAB) are investigated with small-angle neutron scattering (SANS). The roughly spherical DTAB micelle elongates, and the fraction of condensed bromide ions increases, as the DDAB-DTAB mixing ratio increases at a constant total surfactant concentration. The aggregate composition matches that predicted by an ideal mixing model. SANS contrast variation experiments using partially deuterated DTAB (DdTAB) confirm the modeling parameters determined from fully hydrogenated surfactant samples and provide consistent structural information about the interfacial region.