<p>Isoprene is the most abundant unsaturated hydrocarbon in the troposphere. Carbonyl oxides, also called Criegee intermediates, are highly reactive transient species in the ozonolysis of alkenes that play important roles in tropospheric oxidation. Despite decades of efforts, direct observation of Criegee intermediates in isoprene ozonolysis has not been achieved. Here we show the first direct measurement of Criegee intermediates produced in isoprene ozonolysis. Ultra-violet spectra of Criegee intermediates are captured with cavity ringdown spectroscopy and match oscillatory π* ← π transitions of CH₂OO. The in-situ concentration time profiles of Criegee intermediates allow direct kinetic studies and benchmark the isoprene ozonolysis reaction network. The results indicate that CH₂OO dominates stabilized Criegee intermediate chemistry in the low-pressure region, while the role of larger stabilized four-carbon Criegee intermediates could be important in tropospheric oxidation.</p>