Electrical and optical measurements are reported for samples from two undoped semi‐insulating GaAs crystals grown by a vertical molten zone method. The electrical data, taken over the range 290–420 K, included results for samples from both crystals that were so close to intrinsic as to require an ambipolar correction in determining the electron concentration. The compensation balance in this material is controlled by the EL2 midgap defect, of which the fraction ionized depends on trace presence of CAsshallow acceptors, and of shallow donors, probably including SiGa. An increase of the latter towards the tail (top) end of one crystal led to a reduction of the EL2 ionized fraction, and a lowered resistivity—but one still within the conventional semi‐insulating range. Carbon was measured from the strength of its local vibrational mode absorption, while near‐infrared measurements showed that EL2 was present in a concentration ∼1016cm−3, with relatively small variation across a wafer.