Use of the 2,4-di-(tert-butyl)-6-methylphenyl radical permits stabilization of the corresponding dithiophosphorane (I). The analogous diselenophosphorane could not be detected as the monomer. This compound undergoes intramolecular addition of the C-H bond of the ortho-tert-butyl group at the P = Se bond with formation of the corresponding diselenophosphonic acid (III). Intermediate selenophosphine oxide (IV) was trapped as its salt with triethylamine (V).