Hall measurements to 120 K at 50 kbar have been carried out for electrons in the )100), X_1c, satellite valleys in epitaxial and bulk grown GaAs for carrier concentrations in the range 10¹⁴-10¹⁸ cm^-3. Two distinct impurity levels have been found to be associated with the X_1c minima; estimates of activation energies 0.145+or-0.02 eV and 0.066+or-0.01 eV have been obtained for carrier concentrations near 10¹⁵ cm^-3. The activation energies of these levels decreased with increasing carrier concentration. Theoretical analysis of mobility-temperature curves has shown intervalley scattering to be dominant for purer samples while for the heavily doped bulk grown samples space-charge scattering becomes more important. A best estimate of the (X_1c-X_1c) deformation potential field, D_xx, of 0.8*10⁹ eV cm^-1 for a deformation potential E₁ of 5.0 eV has been obtained.