Abstract The compound (Pr i Me 2 Si) 3 CSiMe 2 I, 2 , reacts more readily than (Me 3 Si) 3 CSiMe 2 I, 1 , with electrophiles (AgNO 3 in MeCN; ICl in CH 2 Cl 2 ) but less readily than 1 with the nucleophilic KSCN in MeCN, in keeping with the view that the former type of reaction involves anchimerically assisted formation of an alkyl-bridged cation in the rate-determining step and the latter direct nucleophilic attack at the functional silicon centre. Surprisingly, 2 is more reactive than 1 towards MeOH but less reactive than the latter towards H 2 O-dioxane, adding to the enigma of the mechanism of such solvolyses. The reactions of the labelled compound (Me 3 Si) 3 CSi(CD 3 ) 2 I with MeOH or H 2 O-dioxane take place at the same rate as that of 1 within the experimental uncertainty, and proceed without detectable formation of the rearranged products (Me 3 Si) 2 C[Si(CD 3 ) 2 Me]SiMe 2 OX, X = Me or H, confirming that the solvolyses do not involve cation formation.