High‐purity, lightly Si‐doped (&mgr;77∼70 000–126 000 cm2/V s andn77∼2–8×1014cm−3) molecular beam epitaxy (MBE) GaAs layers have been characterized using variable‐temperature Hall effect andC‐Vmeasurements, photothermal ionization spectroscopy, low‐temperature photoluminescence (PL), and deep level transient spectroscopy (DLTS). The spectroscopic measurements of the residual donors and acceptors indicate that the pronounced increase in carrier concentration which is observed with increasing As flux (for a constant Ga flux) results from incorporation of additional residual S donors from the As source material, and not from reductions in the Si acceptor concentration or residual C acceptor concentration. The increase in carrier concentration with As flux is considerably more pronounced when using an alternative source of As, which introduces both S and 3 additional donor species. The C acceptor concentration increases with As flux using either As source, although the increase is much stronger with the alternative source. The dependence of C concentration on the As source implies that the As source itself contributes at least part of the C background. The Si acceptor concentration is negligible for the range of growth conditions that were used. Close compensation between the residual S donors and C acceptors may account for the high resistivity previously observed in undoped samples grown in this system using the purer As source. The PL data exhibit very weak ‘‘defect’’‐related emissions in the 1.504–1.512 and 1.466–1.482 eV ranges; evidence is presented supporting the existence of a correlation between these two sets of peaks, in agreement with the work of Briones and Collins.Temperature and excitation intensity‐dependent PL measurements are used to demonstrate conclusively that the peaks in the 1.466–1.482 eV range are donor‐to‐acceptor and band‐to‐acceptor in nature, involving normal shallow donors and at least four different acceptor levels whose exact origin is unknown. The ‘‘defect’’ peak intensity is larger in the less pure material which contains more C, implying that the ‘‘defects’’ may be C related. Several electron traps including M1, M3, and M4 are observed in the DLTS spectra, and theC‐Vmeasurements give a total trap concentration of ∼3×1013cm−3.