Electro‐opticgcoefficients, which relate induced birefringence and polarization via the quadratic electro‐optic effect, are found to have the valueg11−g12≈g44≈ 0.011±0.008 m4/C2for 12 oxygen‐octahedra ferroelectrics having Pb in the perovskiteAsite. Measurements on four such ferroelectrics were made using ceramic compositions of the series (Pb, Ba) (Zr, Ti)O3which has a diffuse phase transition and a maximum quadratic electro‐optic coefficient ofR33−R13=3.4×10−16m2/V2at room temperature. To explain why the measuredgis 10 times smaller than those compiled and computed by DiDomenico and Wemple for other oxygen‐octahedra ferroelectrics, the optical‐index dispersion energy &egr;dand the birefringence dispersion constantKwere determined. Values of &egr;d=25 eV andK=−0.5 are as found for the other ferroelectrics. A smallergmust therefore arise from a smaller polarization potential coefficient &bgr;, which is defined with respect to energy shifts and polarization of the BO6perovskite octahedra. Such a definition can lead to a smaller &bgr; in Pb‐containing ferroelectrics because the higher polarizability and displacement of the Pb ion contribute polarization which is not properly accounted for. In any case, thegcoefficient can no longer be considered as a universal constant for oxygen‐octahedra materials.