Cavity ring-down spectroscopy (CRDS) and chemical titration with sulfur dioxide (SO2) were used to indirectly measure the yields of stabilized Criegee intermediates (sCIs) produced from ozonolysis of trans-2-butene and 2,3-dimethyl-2-butene at low pressures in the range of 2-60 Torr. In the case of trans-2-butene, the yield of sCI was found to decrease with decreasing pressure and reach zero at the zero-pressure limit. For 2,3-dimethyl-2-butene, the sCI yield also decreased with decreasing pressure but reached a minimum of 12 +/- 5% at the zero-pressure limit, corresponding to the nascent fraction of CI formed with internal energy below the barriers of dissociation/isomerization. (C) 2017 Elsevier B.V. All rights reserved.