The formation of two different kinds of micelles in the cationic surfactant mixture of HFDePC and CTAC as well as the increased mixing with increasing temperature has been investigated. A critical temperature of demixing was appreciated to 42 degreesC. NMR self-diffusion measurements reveal to us a coexistence of larger fluorocarbon-rich and smaller hydrocarbon-rich micelles, which is also observed using cryo-TEM. We also suggest, from F-19 line width data, that the two surfactant species constituting the demixed micelles are I'microphase-separated". The existence of such aggregates has earlier only been speculated upon. From time-resolved fluorescence quenching measurements, we have estimated the fraction of fluorocarbon surfactant in the hydrocarbon-rich micelles to larger than 0.1.