The electron-impact mass spectra of the title compounds have some important features which give these derivatives certain advantages over the widely studied trimethylsilyl analogues. There is significantly less extensive fragmentation, and abundant ions at (M - t.Bu)+ or (M - i.Pr)+ serve as indicators of molecular weight and should be useful for selected ion monitoring. From various precursors, the ease of elimination of HX2SiOH, via a proposed multi-centre transition state, appears to depend upon conformational and stereochemical factors, as well as the position of the parent silyloxy group, RX2SiO, on the steroid skeleton. This particular fragmentation appears to be a powerful diagnostic method for distinguishing between stereoisomers, being especially useful for differentiation between epimers. In addition, the presence of a 17-silyloxy function promotes a characteristic cleavage of ring B in a skeletally saturated steroid. Elimination of silanol, RX2SiOH, at various stages in the fragmentations of bis-silylated steroids is also an important process, but other familiar features of the spectra of steroid trimethylsilyl ethers, though usually present, are very much suppressed.