Pseudobrookite-type MgTi2O5 (karrooite) is a synthetic crystalline phase with the Bbmm structure and a component in orthorhombic oxide solid solutions, R2Ti2O5-R3+2TiO5, which are present as accessory minerals in lunar and terrestrial rocks. In this study, we present a model for the molar Gibbs free energy of MgTi2O5 as a function of T, P, and the order parameter s = XM1Mg − 2XM2Mg, (−1 ≤ s ≤ 1). We describe the molar Gibbs free energy, (Ḡ), with the equation: Ḡ = ḡ0 + ḡ1· (1 − s) + 3/2 − ḡ2·(1 − s2) − T· S̄config, and take parameter go to represent the molar Gibbs free energy of ordered MgTi2O5 (s = 1), whereas parameters g1 and g2 may represent cation-disorder contributions. We used powder and single-crystal X-ray diffraction, and high-temperature relative enthalpy data, to calibrate the disorder contribution to the volume (bo = 7.3822·10−3 J/bar), and the model parameters g1 (7370.8 J/mol) and g2 (3576.1 J/mol), and heat capacity and volume equation coefficients. We also optimized standard state thermodynamic data from the elements for ordered MgTi2O5, (ΔH0 = −2 498 515.28 J/mol, S0 = 149.55 J/(mol·K), ΔG0 = −2 362 181.72 J/mol, V0 = 5.445 J/bar) consistent with the model parameters and equations, the thermodynamic data in QUILF, and phase-equilibrium experiments involving MgTi2O5, geikelite, rutile, orthoenstatite, and forsterite in the range 973 to 1673 K and 0.0001 to 2.0 GPa. Finally, we investigate theoretically the stability of MgTi2O5 (karrooite) with respect to geikelite, rutile, diopside, enstatite, and forsterite in the CaO-MgO-TiO2-SiO2 system. We find that diopside- and titanite-bearing reactions require extremely high temperatures, and are thus not stable with respect to liquid. The inferred phase relations can be of help in understanding the stability of MgTi2O5 with respect to rutile, geikelite, forsterite, and orthoenstatite, and by extrapolation that of armalcolite relative to rutile, ilmenite, olivine, and orthopyroxene in terrestrial mantle rocks and high-Ti lunar basalts.