The reaction of bis(trimethylsilyl)peroxide with tlithium derivatives of sulphides and nitriles is reported to give the corresponding O-trimethylsilyl hemithioacetals and cyanohydrins. From these products the carbonyl function can be exposed in acidic media or in the presence of fluoride ions. This methodology provides an attractive route to transform a CH2-X group (X = PhS, MeS or CN) into the corresponding