We study the thermotropic phase transition of "salt-free" mixtures of single-tailed cationic and anionic surfactants, called "catanionic systems" in this paper, for concentrated (10 wt %) and diluted (0.5 wt %) samples. Small-angle neutron scattering (SANS) and differential scanning calorimetry (DSC) demonstrate that the chain melting phase transition is identical for diluted and concentrated samples at the same mole fraction (r) of anionic surfactants. The addition of anionic surfactants (for r = 0.5-0.7) induces an increase of the global surface charge, followed by an increase of the main transition temperature. This evolution may e interpreted as the hydrogen bonding formation between carboxylic acid headgroups. At r similar to 0.6, corresponding to the region where facetted vesicles are formed, the "melting" of the alkyl chains is a progressive process extending over 15 degreesC. Here, molten and crystallized bilayers coexist in swollen lamellar phases of catanionic bilayers. This coexistence explains the increase of the viscosity of the solution, via structural reorganization of the bilayers or friction between the partially fused vesicles.