Criegee intermediates (CIs), mainly formed from gas-phase ozonolysis of alkenes, are considered as atmospheric oxidants besides OH and NO3 radicals as well as ozone. Direct CI measurement techniques are inevitably needed for reliable assessment of CIs' role in atmospheric processes. We found that CIs from ozonolysis reactions can be directly probed by means of chemical ionization mass spectrometry with a detection limit of about 10(4)-10(5) molecules cm(-3). Results from quantum chemical calculations support the experimental findings. The simplest CI, CH2OO, is detectable as an adduct with protonated ethers, preferably with protonated tetrahydrofuran. Kinetic measurements yielded k(CH2OO + SO2) = (3.3 +/- 0.9) X 10(-11) and k(CH2OO + acetic acid) = (1.25 +/- 0.30) x 10(-10) cm(3) molecule(-1) s(-1) at 295 +/- 2 K, in very good agreement with recent measurements using diiodomethane photolysis for CH2OO generation. CIs from the ozonolysis of cydohexene, acting as surrogate for cyclic terpenes, are followed as protonated species (CI)H+ using protonated amines as reagent ions. Kinetic investigations indicate a different reactivity of cydohexene-derived CIs compared with that of simple CIs, such as CH2OO. It is supposed that the aldehyde group significantly influences the CI reactivity of the cydohexene-derived CIs. The direct CI detection method presented here should allow study of the formation and reactivity of a wide range of different CIs formed from atmospheric ozonolysis reactions.