The electro-optical properties of the PMN-xPT single-crystal relaxor solid solutions with different PT contents (x = 6, 10, 13%) and the PLMN-25PT transparent ceramic with a La content of 3%, which has been synthesized for the first time, have been studied at room temperature. It has been shown that the dependence of the quadratic electro-optical coefficients on the PT concentration in single crystals passes through a maximum at x ∼ 13%. It has been revealed that, although the PT concentration in the PLMN-25PT ceramic is above 13%, this material exhibits the largest quadratic electro-optical coefficients and the lowest half-wave voltage among all the known relaxor systems. Possible origins of the above phenomena have been discussed.