The thermal and crystallographic properties of tetra-n-butylammonium bromide + tetra-n-butylammonium chloride (TBAB + TBAC) mixed semiclathrate hydrates prepared from mixed aqueous solutions were investigated using phase equilibrium measurements, differential scanning calorimetry (DSC), and powder X-ray diffraction. The equilibrium temperatures gradually vary with both x(guest) {i.e., the mole fraction of guests to (guests + water)} and x(TBAB) {i.e., TBAB to (TBAB + TBAC)}. The maximum equilibrium temperatures for each concentration changed from (x(TBAB) = 0.4 to 0) and were observed at x(TBAB) = 0 below x(guest) = 0.012. The dissociation enthalpies of tetragonal mixed hydrates are between those of the pure hydrates. In the tetragonal pure and mixed hydrates, these results indicate that the thermal stabilities are dependent upon the increase of hydration number by the relaxation of crystal distortion with decreasing x(guest) and x(TBAB). In orthorhombic systems, a single endothermic DSC peak is split to a higher temperature from x(TBAB) = (1.0 to 0.6) at x(guest) = 0.006, and the dissociation enthalpies gradually increase with decreasing x(TBAB). The orthorhombic (TBAB + TBAC) mixed hydrates are formed as eutectic crystals with orthorhombic TBAB hydrate and that they are more thermally stable than the TBAB hydrate.