The volume of Mg2SiO4 wadsleyite has been precisely measured at pressures of 11 to 20 GPa and temperatures of 300 to 2100 K by means of in situ X-ray diffraction in a multi-anvil apparatus. The fixed isothermal bulk modulus K-T0 = 169.2 GPa gives its pressure derivative K-T0' = 4.1(1). The fixed Debye temperature theta(0) = 814 K gives a Gruneisen parameter at ambient pressure gamma(0) = 1.64(2) and its logarithmic volume dependence q = 1.5(1). The pressure derivative of the isothermal bulk modulus, Anderson-Gruneisen parameter and thermal expansion coefficient at ambient pressure are found to be (partial derivative K-T/partial derivative(T))(P) =-0.021(1) GPa/K, delta(T) = 5.5(2), alpha(0) = 2.31(3) x 10(-5) + 1.18(3) x 10(-8) (T - 300) K-1. The pressure and volume dependence of thermal expansion coefficient of wadsleyite is the smallest among those of the mantle minerals. The adiabatic temperature gradient in the upper part of the mantle transition zone is 0.34(1) K/km. Citation: Katsura, T., et al. (2009), P-V-T relations of wadsleyite determined by in situ X-ray diffraction in a large-volume high-pressure apparatus, Geophys. Res. Lett., 36, L11307, doi:10.1029/2009GL038107.