We have carried out neutron transmutation doping of undoped, initially semi-insulating, Czochralski-grown GaAs. We employed a series of nine neutron doses, which added between 2×10¹⁵ and 5×10¹⁷ cm⁻³ Ge and Se to the samples. Electron concentrations determined from Hall effect measurements on annealed samples with the three highest doses agree with the concentration of added Ge and Se within 5%, although the uncertainty in the measurements is 20%. These data are consistent with photoluminescence data which suggest 10% of the amphoteric Ge acts as an acceptor in the annealed material. Room temperature and temperature dependent Hall data place an upper limit of 4.9×10¹⁵ cm⁻³ on the concentration of donors of intermediate depth, consistent with our model for semi-insulating material.