Figure schematically presents the MS detection of different intermediates and products from dicyclopentadiene (R1) and acrylonitrile (R3) reactions in water or acetonitrile microdroplets (Table ). In water microdroplets, the dominant reaction was oxidation, forming the ketone Ox1 from R1 and ketone Ox2 from R2, as recorded in the corresponding mass spectrum (Figure ). These ketones were also characterized by tandem mass spectrometry (Figure ). However, only a minor extent of the DA reaction, relative to oxidation, was observed in the water microdroplets, as evidenced by detecting a trace level of the product DA1. Assuming similar ionization efficiencies for the nitrile compounds (R3 and DA1), the intensity values of these species in the mass spectral data (Figure ) indicate that the yield of the DA reaction in water microdroplets is approximately 0.02%. However, the acetonitrile microdroplets were more effective in facilitating the DA reaction, yielding two products (DA1 and DA2) with a combined yield of approximately 0.7% (Figure ). Given that the average droplet lifetime is around a few hundred microseconds (see Section III in supporting information), these yields, albeit lower in quantity, suggest a more than 10 6 -fold increase of the DA reaction rate in both water and acetonitrile microdroplets compared to the corresponding bulk phase, where the reported rate constant is in the order of 10 -5 M -1 s -1 . 113 (see Section III in Supporting Information). Although oxidation reactions predominated over DA reactions in both water and acetonitrile droplets (Figures ), oxidation efficiency was significantly lower in acetonitrile droplets compared to water droplets. This result suggests that microdroplet-generated hydroxyl radicals or reactive oxygen species may play a key role in driving the oxidation of alkane (R1 and R2) to ketone. A trace amount of water as an impurity in acetonitrile, or the exposure of the acetonitrile droplets to moisture in the air might have driven the observed oxidation reaction (Figure ). Figure presents the histogram compiling the above results, showing that oxidation occurred effectively in water droplets, while the DA reaction was relatively more prominent in acetonitrile droplets. To explore the reasons behind these differences and identify factors that might influence the DA reactions in microdroplets, we assessed the reactions under various spray conditions.