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Phase relations of the olivine‐wadsleyite transition in the system (Mg,Fe)₂SiO₄ have been determined at 1600 and 1900 K using the quench method in a Kawai‐type high‐pressure apparatus. Pressure was determined at a precision better than 0.2 GPa using in situ X‐ray diffraction with MgO as a pressure standard. The transition pressures of the end‐member Mg₂SiO₄ are estimated to be 14.2 and 15.4 GPa at 1600 and 1900 K, respectively. Partition coefficients for Fe and Mg between olivine and wadsleyite are 0.51 at 1600 K and 0.61 at 1900 K. By comparing the depth of the discontinuity with the transition pressure, the temperature at 410 km depth is estimated to be 1760 ± 45 K for a pyrolitic upper mantle. The mantle potential temperature is estimated to be in the range 1550–1650 K. The temperature at the bottom of the upper mantle is estimated to be 1880 ± 50 K. The thickness of the olivine‐wadsleyite …