dc resistance and Hall-effect measurements are conventionally used to study magnetoresistance in lightly doped semiconductors. In the present paper, ac capacitance-voltage (C-V) and conductance-voltage (G-V) measurements on n - type GaAsAlxGa1-xAsn+-type GaAs (hereafter abbreviated as AlGaAs) capacitors are used to study magnetic localization and magnetic freezeout in bulk n - type GaAs at temperatures between 1.4 and 4.2 K and in magnetic fields B up to 14 T. Detailed results are given on two samples whose dopings are 1.7×1015 and 7×1015 cm-3. For the more lightly-doped sample, an exponential magnetoresistance of n - type GaAs is due to magnetic localization. There is good agreement between experimental results and the percolation theory of hopping conduction in lightly doped semiconductors of Shklovskii and Efros, and of Ioselevich. For the more heavily-doped sample, a dip is observed at flat-band voltage in C-V curves for B6 T. The dip is due to magnetic freezeout in n - type GaAs which lowers the carrier concentration and moves the Fermi level into the lower impurity band. Although the mechanisms responsible for magnetoresistance differ for the two samples, the activation energy for magnetoresistance, 3=0.40.6 meV, is the same in the magnetic field region where the data overlap. The results validate the use of C-V and G-V measurements on AlGaAs capacitors for quantitative measurements of magnetoresistance in bulk GaAs. © 1988 The American Physical Society.