The threshold photoelectron spectra and threshold photoelectron photoion coincidence (TPEPICO) mass spectra of methyl t-butyl ether, (CH 3)3COCH3 (MTBE), and methyl trimethylsilyl ether, (CH3)3SiOCH3 (MTMSE), have been measured using synchrotron radiation. The effect of silicon substitution on the unimolecular dissociation processes and the threshold photoelectron spectrum has been investigated. Both molecular ions dissociate at low internal energies. For ionized MTBE, the parent ion is no longer observed at an internal energy of only 0.2 eV. For this reason, it was not possible to fit the TPEPICO data to extract reliable thermochemical information. G3 level calculations place the molecular ion 5 kJ mol-1 above the lowest- energy dissociation products, (CH3)2COCH3+ + CH3, suggesting the participation of an isomer, potentially the distonic ion CH 2(CH3)2CO+(H)CH3, in the dissociation. However, the calculations are not considered accurate enough to reliably determine the role this isomer plays, if any. RRKM modeling of the threshold region of the TPEPICO breakdown curves for ionized MTMSE leads to an E0 for methyl loss of 63 ± 2kJ mol-1, in good agreement with the G3 value of 66 kJ mol-1. The resulting ?fH0 for (CH3)2SiOCH 3+ of 384 ± 10 kJ mol-1 (?fH 298 = 361 ± 10 kJ mol-1)is28kJmol-1 lower than the G3 value of 412 kJ mol-1 due to the G3 ?fH0 for neutral MTMSE being 16 kJ mol-1 higher than the previously reported value and the fact that the experimental IEa is6kJmol-1 lower than the G3 estimate. Appearance energy values for higher- energy fragmentation channels up to 36 (for MTBE) and 32 eV (for MTMSE) are reported and compared to literature values. An investigation of fragment ion peak broadening at high internal energy indicated that the two doubly charged molecular ions are not stable on the microsecond time scale. Each was found to dissociate into two singly charged ions along one or more neutral species. © 2009 American Chemical Society.