The thermodynamic properties of crystalline and liquid KAlSi3O8 are used to calculate the fusion curve of sanidine to 6.5 GPa. New values for the enthalpy and entropy of fusion of sanidine at one bar and 1200 °C (ΔHTf = 63.0 kJ/mol, ΔSTf = 42.8 J/mol-K) are recommended on the basis of improved heat-capacity equations for KAlSi3O8 crystal, glass, and liquid. On the basis of phase-equilibrium experiments on the congruent melting reaction between 2 and 6.5 GPa, the pressure dependence of the liquid compressibility (K0 ’ = dK0/dP, where K0 = 1/β0) is constrained to be 12.2 ± 1.0 in a third-order Birch-Murnaghan equation of state (EOS). The metastable, one-bar melting temperature (Tf ) is additionally constrained to be 1203 ± 26 °C. Determination of the liquid K0 ’ allows the density and compressibility of KAlSi3O8 liquid to be calculated to 6.5 GPa (2.709 ± 0.014 g/cm3 at 1600 °C). The uncertainty in K0 ’ of ±1.0 leads to an error in melt density at 6.5 GPa of ±0.52%. With a K0 ’ = 12.2, the relatively high compressibility of KAlSi3O8 liquid at 1600 °C (K0 = 15.8 GPa) drops rapidly with increasing pressure. The dominant mechanism of compression for KAlSi3O8 liquid between 0 and 6.5 GPa most likely involves topological changes and increases in network connectivity with pressure. It is probable that highly compressible liquids, such as hydrous, silica-rich liquids formed by partial melting of a subducted slab, may have K0 ’ values that exceed 12 (at pressures ≤ 6.5 GPa).