[en] The low-voltage efficiency of the blue-emitting phosphor, cerium activated yttrium silicate (Y_1-mCe_m)₂SiO₅, has been improved by co-activating with gadolinium, (Y_1-m-nCe_mGd_n)₂SiO₅. Gd³⁺ improves the efficiency by transferring energy to Ce³⁺, and makes this phosphor a more promising candidate for low-voltage field emission flat panel displays. Low-voltage cathodoluminescence and photoluminescence measurements were made to determine the optimum concentrations of Gd³⁺ that yielded the most luminous efficient phosphor. For photoluminescence, Ce³⁺ most efficiently luminesces at the excitation wavelength λ_ex=358 nm. Co-activating with Gd³⁺ did not improve the photoluminescent efficiency because Gd³⁺ does not absorb at Ce³⁺ excitation energy, and thus cannot transfer energy. For low-voltage cathodoluminescence, co-activating with Gd³⁺ did improve the efficiency since Gd³⁺ was sufficiently excited, with the optimum composition found to be (Y_0.8425Ce_0.0075Gd_0.15)₂SiO₅