EPR parameters are reported for the first selenoalkyl radicals. They have been prepared by addition of a variety of transient RnM· radicals (M = C, O, S, Si, Sn, etc.) to di-tert-butyl selenoketone. They have the RnMSeC(CMe3)2 structure and, despite the large atomic radius of selenium, they all adopt a conformation in which the RnM group eclipses the Cα 2pz orbital. The EPR parameters are remarkably sensitive to substituent and to temperature. At -50°C, g values range from 2.0005 [RnM = F3C] to 2.0051 [RnM = (n-Bu)3Si], 13Cα hyperfine splittings range from 38.4 [(CH3)3CO] to 53.9 G [(n-Bu)3Si], and 77Se splittings from ≤10 [(n-Bu)3Si] to 66.3 G [(CH3)3CO]. These variations are discussed in terms of the polar effect of the RnM group and in terms of possible changes in the geometry of the radical. The sensitivity of the parameters to RnM suggests that di-tert-butyl selenoketone may have some value as a spin trap.