The reaction of the nitrate TsiSiMe 2ONO 2, 9, (Tsi (Me 3Si) 3C) with 0.25 M solutions of salts MY, viz. NaN 3, CsF, and KSCN, in MeOH at 45°C and 60°C give exclusively the corresponding substitution products TsiSiMe 2Y, but in the case of the halides TsiSiMe 2X, with X I ( 1), Br ( 2), or Cl ( 3), some side-products are formed, mainly the fragmentation product (Me 3Si) 2CHSiMe 2-OMe; the proportion of these side-products increases in the sequence (X ) I < Br < Cl, in line with the decreasing reactivity of the substrate towards the salts. With KOCN, the least reactive of the salts used, substantial amounts of the side-products are formed even from the reactions of the nitrate. The rough relative reactivities of 3, 2, 1, and 9 at 60°C are: 0.1/1.0/4.5/83 towards NaN 3; 0.5/1.0/2.6/115 towards CsF; and 0.13/1.0/4.2/120 towards KSCN. (The rather small spread of rates for the halides is noteworthy.) Examples of rough relative rates of reaction with NaN 3, CsF, and KSCN at 60°C are 0.5/1.0/0.3 for the nitrate and 1.3/1.0/0.45 for the iodide. Sodium methoxide in MeOH, which is ineffective in bringing about substitution of the TsiSiMe 2X compounds, involving attack at the SiMe 2X centre, is roughly comparable with NaN 3 and CsF in bringing about fragmentation, involving attack at an SiMe 3 centre. Features of the substitution reactions are consistent with a rate-determining transition state close to the very crowded five-coordinate silicon species [TsiSiMe 2XY] −. Reaction of the compounds 1–3 and 9 with 0.25 M KSCN in MeCN are some 15–30 times as fast as those in MeOH, and give solely the isothiocyanate TsiSiMe 2NCS.