This study examines the behavior of stearic acid (SA) and 12-hydroxystearic acid (12-HSA) aqueous mixtures as a function of temperature and 12-HSA/SA molar ratio (R). Whatever R, the system exhibits a morphological transition at a given threshold temperature, from multilamellar self-assemblies at low temperature towards small micelles at high temperature, as shown by a combination of transmittance measurements, Wide Angle X-ray diffraction (WAXS), small angle neutron scattering (SANS) and differential scanning calorimetry (DSC) experiments. Such transition originates from the melting of the crystallized fatty acids within the lamellar phase. The precise determination of the threshold temperature, which ranges between 20°C and 50°C depending on R, allows for the construction of the whole phase diagram of the system as a function of R. At high temperature the micelles that are formed are oblate for pure SA solutions (R = 0) and prolate for pure 12-HSA solution (R = 1). In the case of mixtures, there is a progressive continuous transition from oblate to prolate shape when increasing R, with micelles that are almost purely spherical for R = 0.33.