We explored the pressure dependence of acetone oxide (stabilized Criegee Intermediate, sCI) formation from 2,3-dimethyl-2-butene ozonolysis between 50 and 900 Torr using a new, highly accurate technique. We exploited the ability of the sCI to oxidize SO2 to H2SO4, which we measured with a chemical ionization mass spectrometer. We produced the Criegee intermediates (CI) in a high-pressure flow reactor via ozonolysis of 2,3-dimethyl-2-butene (tetramethyl ethylene, TME) and measured the relative H2SO4 concentrations with and without an added OH scavenger. Because the TME reaction with ozone forms acetone oxide (a syn-CI) with unit efficiency, we directly calculated the sCI yields at different pressures from the precisely measured ratio of the uncalibrated H2SO4 signal with and without the scavenger. We observed a linear pressure dependence between 50 and 900 Torr with a minimum stabilization of 12.7 ± 0.6% at 50 Torr and a maximum stabilization of 42 ± 2% at 900 Torr. A linear fit to the measured data points shows a zero-pressure intercept of 15 ± 2%, constraining the fraction of CI formed below the barrier for acetone oxide isomerization.