This work investigates the thermal release of mercury from phosphor powder of spent fluorescent lamps. The treatment conditions and the ability of various reducing agents (primarily sodium borohydride) to lower the overall heating temperature required to improve the release of Hg have been evaluated. Hg species in samples were monitored in a thermal desorption atomic absorption spectrometer system, and total mercury was analyzed in a cold vapor atomic absorption spectrometer. Sodium borohydride was the best reducing agent among the ones studied. However, citric acid presented a high capacity to weaken mercury bonds with the matrix. When the sample was crushed with sodium borohydride for 40min in a mass ratio of 10:1 (sample:reducing agent) and submitted to thermal treatment at 300°C for 2h, the concentration of mercury in a phosphor powder sample with 103mgkg−1 of mercury reached 6.6mgkg−1.