Aggregate transitions in salt-free catanionic surfactant mixtures of tetradecyltrimethylammonium hydroxide (TTAOH)/fatty acid were investigated as a function of surfactant concentration and temperature. Lauric acid (LA), myristic acid (MA), and palmitic acid (PA) were chosen for the current study. The TTAOH/LA mixture exhibited rich phase behavior at room temperature. With increasing total surfactant concentration (c(T)), a bluish vesicular (L-alpha v) phase, an isotropic micellar (L-1) phase, and a birefringent lamellar (L-alpha) phase were observed. Between the L-alpha v phase and the L-1 phase, a narrow L-alpha'/L-1 two-phase region was determined. With increasing temperature, a transition from the L-alpha phase to the L-1 phase was induced at higher c(T) whereas at lower c(T) an opposite transition from the L-1 phase to the L-alpha v phase was noticed. Thus surprisingly, we observed bilayer-to-micelle and micelle-to-bilayer transitions in the same catanionic surfactant system, both induced by the temperature increase. Replacing LA by MA and PA caused a continuous increase in the average Krafft point of the mixture. The L-alpha v-phase region and phase-separated region become larger. Moreover, a single L-1-phase region was absent within the investigated temperature range.