The low-voltage efficiency of the blue-emitting phosphor, cerium activated yttrium silicate (Y 1 - m Ce m ) 2 SiO 5 , has been improved by co-activating with gadolinium, (Y 1 - m - n Ce m Gd n ) 2 SiO 5 . Gd 3 + improves the efficiency by transferring energy to Ce 3 + , 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 Gd 3 + that yielded the most luminous efficient phosphor. For photoluminescence, Ce 3 + most efficiently luminesces at the excitation wavelength λ e x =358nm. Co-activating with Gd 3 + did not improve the photoluminescent efficiency because Gd 3 + does not absorb at Ce 3 + excitation energy, and thus cannot transfer energy. For low-voltage cathodoluminescence, co-activating with Gd 3 + did improve the efficiency since Gd 3 + was sufficiently excited, with the optimum composition found to be (Y 0 . 8 4 2 5 Ce 0 . 0 0 7 5 Gd 0 . 1 5 ) 2 SiO 5 .