Physical mechanisms causing the refractive index increase &Dgr;nin uniaxial oxidized ferroelectric crystals subjected to double doping are discussed. The analysis shows that the index changes of the two different dopants, &Dgr;nAand &Dgr;nB, are additive only if the corresponding changes of the spontaneous polarization are small. Ti indiffusion and proton exchange (TIPE) waveguides in LiNbO3are considered as an example of the case when this condition is not fulfilled. The hydrogen–lithium substitution in such waveguides leads to a strong reduction of the spontaneous polarization of the virgin crystals and the final index change &Dgr;nis not a sum of &Dgr;nAand &Dgr;nB. A physical model is developed to explain light waveguiding in double‐doped ferroelectric structures. Experimental data obtained for TIPE waveguides confirm the model. ©1995 American Institute of Physics.