The full set of single-crystal elastic moduli of forsterite (α-Mg2SiO4) and wadsleyite (β-Mg2SiO4) were measured at pressures of 3-16 GPa and 0-14 GPa, respectively. For forsterite, the pressure derivatives of the bulk (K0‘s) and shear (G0') modulus are 4.2 ± 0.2 and 1.4 ± 0.1, respectively. For wadsleyite, the corresponding pressure derivatives are 4.3 ± 0.2 and 1.4 ± 0.2. These values are much lower than those reported in earlier, low-pressure studies for both materials. At the pressure of the 410-km seismic discontinuity, the room-temperature velocity increase across the a-p transition is 9.8% for compressional waves and 12.4% for shear waves. The results are consistent with an olivine fraction in the upper mantle of 30-50%.