The rate of hydrogen-deuterium exchange (HDX) in aqueous droplets of phenethylamine has been determined with submillisecond temporal resolution by mass spectrometry using nanoelectrospray ionization with a theta-capillary. The average speed of the micro droplets is measured using microparticle image velocimetry. The droplet travel time is varied from 20 to 320 mu s by changing the distance between the emitter and the heated inlet to the mass spectrometer and the voltage applied to the emitter source. The droplets were found to accelerate by similar to 30% during their observable travel time. Our droplet imaging shows that the theta-capillary produces two Taylor cone-jets (one per channel), causing mixing to take place from droplet fusion in the Taylor spray zone. Phenethylamine (phi CH2CH2NH2) was chosen to study because it has only one functional group (-NH2) that undergoes rapid HDX. We model the HDX with a system of ordinary differential equations. The rate constant for the formation of -NH2D+ from -NH3+ is 3660 +/- 290 s(-1), and the rate constant for the formation of -NHD2+ from -NH2D+ is 3330 +/- 270 s(-1). The observed rates are about 3 times faster than what has been reported for rapidly exchangeable peptide side-chain groups in bulk measurements using stopped-flow kinetics and NMR spectroscopy. We also applied this technique to determine the HDX rates for a small 10-residue peptide, angiotensin I, in aqueous droplets, from which we found a 7-fold acceleration of HDX in the droplet compared to that in bulk solution.