Optical absorption, electron spin resonance (ESR), thermoluminescence, and x‐ray‐induced luminescence techniques have been used to characterize point defects in LiNbO3. A series of crystals with different magnesium‐doping levels and Li/Nb ratios were investigated, and the defects were introduced either by x‐ray irradiation at low temperatures (10–85 K) or by reduction at high temperature in a vacuum. The samples may be classified into two groups, according to their response to radiation and reduction. A threshold (i.e., a sharp change in behavior due to a small change in composition) marks the change from one type of response to the other. The concentrations of both magnesium and lithium affect the threshold level. An electron trap, represented by an optical absorption band peaking at 1200 nm and a broad ESR spectrum centered atgc=1.82, is observed in several of the heavily‐doped crystals after irradiation. A corresponding hole trap produced during the same irradiation has an optical absorption peak near 500 nm and an ESR line atgc=2.03. An intense thermoluminescence peak, obtained after x‐ray irradiation at 15 K, occurs at 70 K in this latter group of crystals and correlates with the thermal decay of the electron and hole traps just described. Following vacuum reduction at 1000 °C, these heavily‐doped crystals exhibit an optical absorption spectrum that can be decomposed into two bands peaking near 760 and 1200 nm, and a broad ESR spectrum withgc=1.82. The 1200‐nm band and ESR signal are associated with an electron trap (identical to the one produced during the irradiations). This electron trap is suggested to be a Mg+complex.