The Al2O solubility in orthopyroxene (OPx) coexisting with pyrope has been determined at several P-T conditions in the system MgO-Al2O3-SiO2 ( (MAS), using synthetic crystalline starting mixtures and approaching from both high- and low-alumina sides. These results and the reversed experimental data of Danckwerth and Newton (1978) on Al-OPx+spinel+forsterite equilibrium have been treated thermodynamically, using the solution model of Al-OPx proposed by Ganguly and Ghose (1979), to express Al2O3 solubility in OPx as a function of P and T in both assemblages. The calculated alumina isopleths are in very good agreement with all available reversed experimental data. In addition, the univariant boundary between spinel peridotite and garnet peridotite in the MAS system, which is defined by the intersection of complementary alumina isopleths in these two fields, is in excellent agreement with experimental reversals of the boundary of Danckwerth and Newton. The spinel field isopleths have somewhat steeper dT/dP slopes than those calculated by previous workers but are sufficiently sensitive to pressure to be useful as a practical geobarometer. The garnet field isopleths are nearly equally sensitive to both P and T but are somewhat different from those of MacGregor (1974) and show mild curvature, concave toward the pressure axis. The univariant boundary shows moderate curvature, convex toward the temperature axis, with a pressure minimum of 19 kbar at 840¿C. The 'pyroxene geotherm' of Boyd (1973) has been reexamined in the light of the results of this study and the garnet-clinopyroxene geothermometer of Ganguly (1979). Boyd's conclusion that the sheared garnet iherzolite nodules in kimberlite pipes have formed at higher P-T conditions than the granular ones is reinforced: however, the inferred P-T conditions do not seem to suggest an inflection of the geotherm between the two groups of nodules. The pyroxene or nodule geotherm overlaps the shield geotherm of Clark and Ringwood but progressively diverges from the latter toward higher temperature with increasing depth.