Reactions of Hg(SeRf)2 (Rf = CF3, C2F5, C3F7) with Et2AlI or AlI3 in octamethylcyclotetrasiloxane produce the unstable RfC(Se)F [I; Rf = CF3 (II), C2F5 (III), CF3Se]. The Se compds. are very reactive and polymerize to rubberlike products. On heating, the polymers decomp. almost quant. to the monomers or dimers. In CFCl3 soln. I dimerize at 20 Deg in sunlight to the corresponding cis/trans-1,3-diselenetanes IV. The structure of IV (R = R2 = CF3, R1 = R3 = F) is detd. by x-ray anal. With cyclopentadiene, II and III react to form the [4 + 2] cycloaddn. products. Halogen exchange reactions take place between IV and BCl3. The cis isomers react much faster than the trans isomers to give a mixt. of cis- and trans-IV (R = R2 = Cl, R1 = R3 = Rf; R = R3 = Cl, R1 = R2 = Rf). Fluorine abstraction with formation of 1,3-diselenetan-2-ylium ions is accomplished by reactions of IV with AsF5 in SO2.