Radioluminescence of float-zone grown single-crystal Lu 2 SiO 5 :RE (RE=Tb, Sm, Tm, Eu, Pr, Yb) has been measured and prominent electronic transitions have been identified. Tb, Sm and Tm are excellent host-lattice activators that yield strong emission relative to the luminescence from well-known Lu 2 SiO 5 :Ce. Eu- and Pr-doped Lu 2 SiO 5 exhibit weak luminescence whereas the Yb-doped sample is non-luminescent in the UV–vis portion of the spectrum. With the possible exception of Lu 2 SiO 5 :Pr, Tb 3+ is found to be an unintentional impurity in all samples investigated. Lu 2 SiO 5 :Sm is a candidate phosphor for radiation detection because it is dense and bright with orange–red emission that matches the spectral response of high-efficiency photodiodes.