We show here conclusively that the internal field originates from nonstoichiometric point defects inLiNbO3crystals. The switching fields required for 180° domain reversal in congruent crystals[C=Li2O/(Li2O+Nb2O5)=0.484]are ∼4–5 times larger than the switching fields for nearstoichiometric crystals(C=0.498).An internal field of ∼2.5 kV/mm observed in congruent crystals disappears in stoichiometric crystals. The concentration of hydrogen incorporated during crystal growth has no effect on the switching or internal fields. The measured spontaneous polarization,Ps=80±5&mgr;C/cm2is relatively insensitive to the crystal nonstoichiometry and the hydrogen content. ©1998 American Institute of Physics.